Lithium-sulfur battery containing two anode-protecting layers

ABSTRACT

Provided is a rechargeable alkali metal-sulfur cell comprising: (a) an anode; (b) a cathode active material layer comprising a sulfur-containing material; and (c) an electrolyte or an electrolyte/separator layer; wherein the anode comprises (i) an anode active material layer; (ii) a first anode-protecting layer, in physical contact with the anode active material layer, having a thickness from 1 nm to 100 μm and comprising a thin layer of an electron-conducting material having a specific surface area greater than 50 m 2 /g; and (iii) a second anode-protecting layer in physical contact with the first anode-protecting layer, having a thickness from 1 nm to 100 μm and comprising an elastomer having a fully recoverable tensile elastic strain from 2% to 1,000% and a lithium ion conductivity from 10 −8  S/cm to 5×10 −2  S/cm when measure at room temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 16/010,965, filed Jun. 18, 2018 and U.S. patentapplication Ser. No. 16/116,329, filed Aug. 29, 2018, which are herebyincorporated by reference for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underW56KGU-18-C-0012 awarded by the DOD. The United States Government hascertain rights in this invention.

FIELD OF THE INVENTION

The present invention provides a protective layer for use in a secondaryor rechargeable alkali metal-sulfur battery, including thelithium-sulfur battery, sodium-sulfur battery, and potassium-sulfurbattery.

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur and Li metal-air batteries) are considered promising powersources for electric vehicle (EV), hybrid electric vehicle (HEV), andportable electronic devices, such as lap-top computers and mobilephones. Lithium as a metal element has the highest capacity (3,861mAh/g) compared to any other metal or metal-intercalated compound as ananode active material (except Li_(4.4)Si, which has a specific capacityof 4,200 mAh/g). Hence, in general, Li metal batteries have asignificantly higher energy density than lithium ion batteries.

One of the most promising energy storage devices is the lithium-sulfur(Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and thatof S is 1,675 mAh/g. In its simplest form, a Li—S cell consists ofelemental sulfur as the positive electrode and lithium as the negativeelectrode. The lithium-sulfur cell operates with a redox couple,described by the reaction S₈+16Li↔8Li₂S that lies near 2.2 V withrespect to Li⁺/Li^(o). This electrochemical potential is approximately2/3 of that exhibited by conventional positive electrodes (e.g. LiMnO₄).However, this shortcoming is offset by the very high theoreticalcapacities of both Li and S. Thus, compared with conventionalintercalation-based Li-ion batteries, Li—S cells have the opportunity toprovide a significantly higher energy density (a product of capacity andvoltage). Assuming complete reaction to Li₂S, energy densities valuescan approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on thecombined Li and S weight or volume. If based on the total cell weight orvolume, the energy densities of optimally designed Li—S cellconfigurations can reach approximately 1,000 Wh/kg and 1,100 Wh/l,respectively. However, the current Li-sulfur cells reported by industryleaders in sulfur cathode technology have a maximum cell specific energyof 250-400 Wh/kg (based on the total cell weight), which is far belowwhat is possible.

In summary, despite its considerable advantages, the Li—S cell isplagued with several major technical problems that have thus farhindered its widespread commercialization:

-   (1) Conventional lithium metal cells still have dendrite formation    and related internal shorting issues.-   (2) Sulfur or sulfur-containing organic compounds are highly    insulating, both electrically and ionically. To enable a reversible    electrochemical reaction at high current densities or    charge/discharge rates, the sulfur must maintain intimate contact    with an electrically conductive additive. Various carbon-sulfur    composites have been utilized for this purpose, but only with    limited success owing to the limited scale of the contact area.    Typical reported capacities are between 300 and 550 mAh/g (based on    the cathode carbon-sulfur composite weight) at moderate rates.-   (3) The cell tends to exhibit significant capacity decay during    discharge-charge cycling. This is mainly due to the high solubility    of the lithium polysulfide anions formed as reaction intermediates    during both discharge and charge processes in the polar organic    solvents used in electrolytes. During cycling, the lithium    polysulfide anions can migrate through the separator to the Li    negative electrode whereupon they are reduced to solid precipitates    (Li₂S₂ and/or Li₂S), causing active mass loss. In addition, the    solid product that precipitates on the surface of the positive    electrode during discharge becomes electrochemically irreversible,    which also contributes to active mass loss.-   (4) More generally speaking, a significant drawback with cells    containing cathodes comprising elemental sulfur, organosulfur and    carbon-sulfur materials relates to the dissolution and excessive    out-diffusion of soluble sulfides, polysulfides, organo-sulfides,    carbon-sulfides and/or carbon-polysulfides (hereinafter referred to    as anionic reduction products) from the cathode into the rest of the    cell. This phenomenon is commonly referred to as the Shuttle Effect.    This process leads to several problems: high self-discharge rates,    loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.

In response to these challenges, new electrolytes, protective films forthe lithium anode, and solid electrolytes have been developed. Someinteresting cathode developments have been reported recently to containlithium polysulfides; but, their performance still fall short of what isrequired for practical applications. Despite the various approachesproposed for the fabrication of high energy density Li—S cells, thereremains a need for cathode materials, production processes, and celloperation methods that retard the out-diffusion of S or lithiumpolysulfide from the cathode compartments into other components in thesecells, improve the utilization of electro-active cathode materials (Sutilization efficiency), and provide rechargeable Li—S cells with highcapacities over a large number of cycles.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials(except pure silicon, but silicon has pulverization issues). Lithiummetal would be an ideal anode material in a lithium-sulfur secondarybattery if dendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the sulfur cathode in room temperaturesodium-sulfur cells (RT Na—S batteries) or potassium-sulfur cells (K—S)face the same issues observed in Li—S batteries, such as: (i) low activematerial utilization rate, (ii) poor cycle life, and (iii) low Coulombicefficiency. Again, these drawbacks arise mainly from insulating natureof S, dissolution of S and Na or K polysulfide intermediates in liquidelectrolytes (and related Shuttle effect), and large volume changeduring charge/discharge.

Hence, an object of the present invention is to provide a rechargeablealkali metal battery (e.g., Li—S, Na—S, and K—S battery) that exhibitsan exceptionally high specific energy or high energy density. Oneparticular technical goal of the present invention is to provide analkali metal-sulfur or alkali ion-sulfur cell with a cell specificenergy greater than 400 Wh/Kg, preferably greater than 500 Wh/Kg, andmore preferably greater than 600 Wh/kg (all based on the total cellweight).

Another object of the present invention is to provide an alkalimetal-sulfur cell that exhibits a high cathode specific capacity (higherthan 1,200 mAh/g based on the sulfur weight, or higher than 1,000 mAh/gbased on the cathode composite weight, including sulfur, conductingadditive or substrate, and binder weights combined, but excluding theweight of cathode current collector). The specific capacity ispreferably higher than 1,400 mAh/g based on the sulfur weight alone orhigher than 1,200 mAh/g based on the cathode composite weight. This mustbe accompanied by a high specific energy, good resistance to dendriteformation, and a long and stable cycle life.

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the sulfur or lithium polysulfideweight alone (not the total cathode composite weight), but unfortunatelya large proportion of non-active materials (those not capable of storinglithium, such as conductive additive and binder) is typically used intheir Li—S cells. For practical use purposes, it is more meaningful touse the cathode composite weight-based capacity value.

A specific object of the present invention is to provide a rechargeablealkali metal-sulfur cell based on rational materials and battery designsthat overcome or significantly reduce the following issues commonlyassociated with conventional Li—S and room temperature Na—S cells: (a)dendrite formation (internal shorting); (b) extremely low electric andionic conductivities of sulfur, requiring large proportion (typically30-55%) of non-active conductive fillers and having significantproportion of non-accessible or non-reachable sulfur or alkali metalpolysulfides); (c) poor sulfur utilization efficiency; (d) dissolutionof S and alkali metal polysulfide in electrolyte; (e) migration ofalkali metal polysulfides from the cathode to the anode (whichirreversibly react with Li, Na, or K at the anode), resulting in activematerial loss and capacity decay (the shuttle effect); and (f) shortcycle life.

SUMMARY OF THE INVENTION

The present invention provides an alkali metal-sulfur cell (e.g.lithium-sulfur cell, sodium-sulfur cell, and potassium-sulfur cell). Thelithium-sulfur battery can include the lithium metal-sulfur battery(having lithium metal as the anode active material and sulfur as thecathode active material) and the lithium ion-sulfur battery (e.g.prelithiated Si or graphite as the anode active material and sulfur asthe cathode active material). The sodium-sulfur battery can include thesodium metal-sulfur battery (having sodium metal as the anode activematerial and sulfur as the cathode active material) and the sodiumion-sulfur battery (e.g. hard carbon as the anode active material andsulfur as the cathode active material).

In some embodiments, the alkali metal-sulfur cell (selected fromlithium-sulfur cell, sodium-sulfur cell, or potassium-sulfur cell, saidalkali metal-sulfur cell) comprises: (a) an anode; (b) a cathode activematerial layer, comprising a sulfur-containing material selected from asulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid,conducting polymer-sulfur hybrid, metal sulfide, sulfur compound, or acombination thereof, and an optional cathode current collectorsupporting said cathode active material layer; and (c) an electrolyte orelectrolyte/separator layer; wherein the anode comprises:

-   -   i) an anode active material layer containing a layer of lithium,        sodium, potassium, a lithium alloy, a sodium alloy, a potassium        alloy, a lithium-absorbing compound, a sodium-absorbing        compound, or a potassium-absorbing compound, as an anode active        material and an optional anode current collector supporting the        anode active material layer;    -   ii) a first anode-protecting layer having a thickness from 1 nm        to 100 μm and comprising a thin layer of electron-conducting        material selected from graphene sheets, carbon nanotubes, carbon        nanofibers, carbon or graphite fibers, expanded graphite flakes,        metal nanowires, conductive polymer fibers, or a combination        thereof, wherein said first anode-protecting layer has a        specific surface area greater than 50 m²/g and is in physical        contact with the anode active material layer; and    -   iii) a second anode-protecting layer in physical contact with        the first anode-protecting layer, having a thickness from 1 nm        to 100 μm and comprising an elastomer having a fully recoverable        tensile elastic strain from 2% to 1,000% and a lithium ion        conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm when measure at room        temperature.

In certain embodiments, the anode comprises an anode active materiallayer that comprises a layer of lithium or lithium alloy (in the form ofa foil, coating, or multiple particles aggregated together) as an anodeactive material. The foil or coating of lithium or lithium alloy may besupported by a current collector (e.g. a Cu foil, a Ni foam, a porouslayer of nanofilaments, such as graphene sheets, carbon nanofibers,carbon nanotubes, etc.). The anode may contain Na or K foil, coating orparticles for Na—S or K—S cells, respectively.

In some embodiments, the lithium battery does not contain a porousseparator and the second anode-protecting layer itself acts as aseparator that electronically separates the anode active material layerfrom the cathode.

The first anode-protecting layer, being electron-conducting and having ahigh specific surface area (preferably >50 m²/g) can significantlydecrease the exchange current density imposed on the anode activematerial (the Li metal), to the extent that presumably the localexchange current density can be lower than the threshold exchangecurrent density for lithium dendrite initiation or that for the dendritepropagation, once initiated.

Preferably, the first anode-protecting layer contains a thin membrane,paper, non-woven, woven fabric, etc. of graphene sheets, carbonnanotubes, carbon nanofibers, carbon or graphite fibers, expandedgraphite flakes, metal nanowires, conductive polymer fibers, or acombination thereof. This layer must be reasonably permeable to lithiumions; e.g. having pores to allow for easy migration of lithium ions.

The elastomer (sulfonated or non-sulfonated), in the secondanode-protecting layer, is a high-elasticity material which exhibits anelastic deformation of at least 2% (preferably at least 5% and up toapproximately 1,000%) when measured under uniaxial tension. In the fieldof materials science and engineering, the “elastic deformation” isdefined as a deformation of a material (when being mechanicallystressed) that is essentially fully recoverable upon release of the loadand the recovery process is essentially instantaneous (no or little timedelay). The elastic deformation is more preferably greater than 10%,even more preferably greater than 30%, further more preferably greaterthan 50%, and still more preferably greater than 100%. In someembodiments, the elastomer preferably and more typically has a fullyrecoverable tensile strain from 10% to 500%, a thickness from 10 nm to20 μm, a lithium ion conductivity of at least 10⁻⁵ S/cm, and anelectrical conductivity of at least 10⁻³ S/cm when measured at roomtemperature on a cast thin film 20 μm thick.

Preferably, the elastomer contains a sulfonated or non-sulfonatedversion of an elastomer selected from natural polyisoprene, syntheticpolyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butylrubber, styrene-butadiene rubber, nitrile rubber, ethylene propylenerubber, ethylene propylene diene rubber, metallocene-basedpoly(ethylene-co-octene) (POE) elastomer, poly(ethylene-co-butene) (PBE)elastomer, styrene-ethylene-butadiene-styrene (SEBS) elastomer,epichlorohydrin rubber, polyacrylic rubber, silicone rubber,fluorosilicone rubber, perfluoroelastomers, polyether block amides,chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplasticelastomer, protein resilin, protein elastin, ethyleneoxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer,or a combination thereof.

These elastomers or rubbers, when present without graphene sheets,exhibit a high elasticity (having a fully recoverable tensile strainfrom 2% to 1,000%). In other words, they can be stretched up to 1,000%(10 times of the original length when under tension) and, upon releaseof the tensile stress, they can fully recover back to the originaldimension. By adding from 0.01% to 50% by weight of a conductivereinforcement material and/or a lithium ion-conducting species dispersedin a sulfonated elastomeric matrix material, the fully recoverabletensile strains are typically reduced down to 2%-500% (more typicallyfrom 5% to 300% and most typically from 10% to 150%).

The elastomer, if sulfonated, becomes significantly more lithiumion-conducting. The lithium ion conductivity of an elastomer, sulfonatedor un-sulfonated, may be further improved if some desired amount oflithium ion-conducting additive is incorporated into the elastomermatrix.

The conducting material in the first anode-protecting layer (or as areinforcement material in the second anode protecting layer) ispreferably in a nano filamentary or nanosheet-like form, such as ananotube, nanofiber, nanowire, nanoplatelet, or nanodisc. In someembodiments, the conductive reinforcement material is selected fromgraphene sheets, carbon nanotubes, carbon nanofibers, carbon or graphitefibers, expanded graphite flakes, metal nanowires, conductive polymerfibers, or a combination thereof. These electron-conducting materialsare preferably made into a form of paper sheet, porous membrane, fabric,nonwoven, etc. having pores to allow lithium ions to transport through.

The graphene sheets are preferably selected from pristine graphene,graphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, nitrogenated graphene, hydrogenated graphene, doped graphene,functionalized graphene, or a combination thereof. The graphene sheetspreferably comprise single-layer graphene or few-layer graphene, whereinthe few-layer graphene is defined as a graphene platelet formed of lessthan 10 graphene planes. The carbon nanotubes (CNTs) can be asingle-walled CNT or multi-walled CNT. The carbon nanofibers may bevapor-grown carbon nanofibers or electrospinning based carbon nanofibers(e.g. electrospun polymer nanofibers that are subsequently carbonized).

In certain embodiments, the electrically conducting material in thefirst anode-protecting layer may be selected from an electron-conductingpolymer, a metal particle or wire (or metal nanowire), a graphene sheet,a carbon fiber, a graphite fiber, a carbon nanofiber, a graphitenanofiber, a carbon nanotube, a graphite particle, an expanded graphiteflake, an acetylene black particle, or a combination thereof. Theelectrically conducting material (e.g. metal nanowire, nanofiber, etc.)preferably has a thickness or diameter less than 100 nm.

The first anode-protecting layer may be a thin film (thin paper,membrane, fabric, etc.) disposed against a lithium foil/coating layersurface. The second anode-protecting layer is in turn a thin film orcoating of an elastomer disposed against the first anode-protectinglayer. The first anode-protecting layer, being electrically conductingand having a high specific surface area (preferably >50 m²/g, morepreferably >100 m²/g, further more preferably >200 m²/g, even morepreferably >500 m²/g, and most preferably >1,000 m²/g), helps to reduceor eliminate the formation of lithium metal dendrite, likely due to asignificantly reduced exchange current density at the anode. This firstprotecting layer also appears to enable a uniform deposition of lithiumions during battery recharge.

The sulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphenehybrid, or conducting polymer-sulfur hybrid may be a mixture, blend,composite, or chemically or physically bonded entity of sulfur orsulfide with a carbon, graphite, graphene, or conducting polymermaterial. For instance, a sulfur-graphene hybrid can be a simple mixture(in a particle form) of sulfur and graphene prepared by ball-milling.Such a hybrid can contain sulfur bonded on surfaces of a graphene oxidesheet, etc. As another example, the sulfur-carbon hybrid can be a simplemixture (in a particle form) of sulfur and carbon nanotubes, or cancontain sulfur residing in pores of activated carbon particles.

In the rechargeable alkali metal-sulfur cell, the metal sulfide maycontain a material denoted by M_(x)S_(y), wherein x is an integer from 1to 3 and y is an integer from 1 to 10, and M is a metal element selectedfrom an alkali metal, an alkaline metal selected from Mg or Ca, atransition metal, a metal from groups 13 to 17 of the periodic table, ora combination thereof. The metal element M preferably is selected fromLi, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al. In some preferredembodiments, the metal sulfide in the cathode layer contains Li₂S₁,Li₂S₂, Li₂S₃, Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S₁,Na₂S₂, Na₂S₃, Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S₁,K₂S₂, K₂S₃, K₂S₄, K₂S₅, K₂S₆, K₂S₇, K₂Ss, K₂S₉, or K₂S₁₀.

In the rechargeable alkali metal-sulfur cell, the carbon or graphitematerial in the cathode active material layer may be selected frommesophase pitch, mesophase carbon, mesocarbon micro-bead (MCMB), cokeparticle, expanded graphite flake, artificial graphite particle, naturalgraphite particle, highly oriented pyrolytic graphite, soft carbonparticle, hard carbon particle, carbon nanotube, carbon nanofiber,carbon fiber, graphite nanofiber, graphite fiber, carbonized polymerfiber, activated carbon, carbon black, or a combination thereof. Thegraphene may be selected from pristine graphene, graphene oxide, reducedgraphene oxide (RGO), graphene fluoride, nitrogenated graphene,hydrogenated graphene, doped graphene, functionalized graphene, or acombination thereof.

In certain embodiments, the electrically conducting material may beselected from an electron-conducting polymer, a metal particle or wire(or metal nanowire), a graphene sheet, a carbon fiber, a graphite fiber,a carbon nanofiber, a graphite nanofiber, a carbon nanotube, a graphiteparticle, an expanded graphite flake, an acetylene black particle, or acombination thereof. The electrically conducting material (e.g. metalnanowire, nanofiber, etc.) preferably has a thickness or diameter lessthan 100 nm.

It may be noted that lithium foil/coating layer may decrease inthickness due to dissolution of lithium into the electrolyte to becomelithium ions as the lithium battery is discharged, creating a gapbetween the current collector and the protective layer if the protectivelayer were not elastic. Such a gap would make the re-deposition oflithium ions back to the anode impossible. We have observed that theinstant elastomer layer is capable of expanding or shrinking congruentlyor conformably with the anode layer covered by the first protectinglayer of an electron-conducting material. This capability helps tomaintain a good contact between the current collector (or the lithiumfilm itself) and the protective layers, enabling the re-deposition oflithium ions without interruption.

At the anode side, preferably and typically, the elastomer for thesecond protective layer is designed or selected to have a lithium ionconductivity no less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴S/cm, and most preferably no less than 10⁻³S/cm. Some of the selectedelastomers, when sulfonated, can exhibit a lithium-ion conductivitygreater than 10⁻² S/cm. In some embodiments, the elastomer is anelastomer containing no additive or filler dispersed therein. In others,the elastomer composite is an elastomer matrix composite containing from0.1% to 40% by weight (preferably from 1% to 30% by weight) of a lithiumion-conducting additive dispersed in an elastomer matrix material. Insome embodiments, the elastomer contains from 0.1% by weight to 10% byweight of a reinforcement nanofilament selected from carbon nanotube,carbon nanofiber, graphene, or a combination thereof.

In certain embodiments, the elastomer further contains from 0.1% to 40%by weight of a lithium ion-conducting additive dispersed therein. Incertain embodiments, the lithium ion-conducting may be selected fromLi₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X=F,Cl, I, or Br, R=a hydrocarbon group, 0≤x≤1 and 1≤y≤4. In certainembodiments, the lithium ion- or sodium ion-conducting may be selectedfrom Na₂CO₃, Na₂O, Na₂C₂O₄, NaOH, NaX, ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂,(CH₂OCO₂Na)₂, Na₂S,Na_(x)SO_(y), or a combination thereof, wherein X=F,Cl, I, or Br, R=a hydrocarbon group, 0≤x≤1 and 1≤y≤4.

In certain embodiments, the lithium ion-conducting additive or sodiumion-conducting additive is selected from the following lithium salts ortheir sodium salt counterparts: lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂),lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI),lithium bis(trifluoromethanesulfonyl)imide, lithiumbis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid-based lithium salt, or a combination thereof.

In certain preferred embodiments, the electron-conducting polymer in theconducting polymer-sulfur hybrid is selected from polyaniline,polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, a sulfonatedderivative thereof, or a combination thereof.

In certain embodiments, the elastomer forms a mixture or blend with alithium ion-conducting polymer selected from a lower molecular weight(<500,000 g/mole) version of poly(ethylene oxide) (PEO), polypropyleneoxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate)(PMMA), poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, or a combination thereof.

The present invention enables the rechargeable alkali metal-sulfur cellto deliver a sulfur utilization efficiency from 80% to 99%, moretypically from 85% to 97%.

In the rechargeable alkali metal-sulfur cell, the electrolyte may beselected from polymer electrolyte, polymer gel electrolyte, compositeelectrolyte, ionic liquid electrolyte, aqueous electrolyte, organicliquid electrolyte, soft matter phase electrolyte, solid-stateelectrolyte, or a combination thereof. The electrolyte may contain asalt selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂,lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-methanesulfonate (NaCF₃SO₃),potassium trifluoro-methanesulfonate (KCF₃SO₃), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide(NaTFSI), bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), ora combination thereof.

The electrolyte may contain a solvent selected from ethylene carbonate(EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethylcarbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xyleneor methyl acetate (MA), fluoroethylene carbonate (FEC), vinylenecarbonate (VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, roomtemperature ionic liquid, or a combination thereof.

In some embodiments, the rechargeable alkali metal-sulfur cell furthercomprises one or two cathode-protecting layers disposed between thecathode active material layer and the separator/electrolyte layerwherein the cathode-protecting layers are selected from:

-   -   a) a first cathode-protecting layer having a thickness from 1 nm        to 100 μm and comprising a thin layer of electron-conducting        material selected from graphene sheets, carbon nanotubes, carbon        nanofibers, carbon or graphite fibers, expanded graphite flakes,        metal nanowires, conductive polymer fibers, or a combination        thereof, wherein said first anode-protecting layer has a        specific surface area greater than 50 m²/g and is in physical        contact with the cathode active material layer or the        separator/electrolyte layer; and/or    -   b) a second cathode-protecting layer in physical contact with        the first cathode-protecting layer, having a thickness from 1 nm        to 100 μm and comprising an elastomer having a fully recoverable        tensile elastic strain from 2% to 1,000% and a lithium ion        conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm when measure at room        temperature.

In certain embodiments, the anode active material layer of the inventedcell contains an anode active material selected from lithium metal,sodium metal, potassium metal, a lithium metal alloy, sodium metalalloy, potassium metal alloy, a lithium intercalation compound, a sodiumintercalation compound, a potassium intercalation compound, a lithiatedcompound, a sodiated compound, a potassium-doped compound, lithiatedtitanium dioxide, lithium titanate, lithium manganate, a lithiumtransition metal oxide, Li₄TisO₁₂, or a combination thereof.

The rechargeable alkali metal-sulfur cell may be a lithium ion-sulfurcell and, in this case, the anode active material layer contains ananode active material (a lithium-absorbing compound) selected from thegroup consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead(Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel(Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium(Cd), and lithiated versions thereof; (b) alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements,and lithiated versions thereof, wherein said alloys or compounds arestoichiometric or non-stoichiometric; (c) oxides, carbides, nitrides,sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites,and lithiated versions thereof; (d) salts and hydroxides of Sn andlithiated versions thereof; (e) carbon or graphite materials andprelithiated versions thereof; and combinations thereof.

The rechargeable alkali metal-sulfur cell may be a sodium ion-sulfurcell or potassium ion-sulfur cell and, in this case, the anode activematerial layer contains an anode active material (a sodium-absorbingcompound) selected from the group consisting of: (a) sodium- orpotassium-doped silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti),cobalt (Co), nickel (Ni), manganese (Mn), cadmium (Cd), and mixturesthereof; (b) sodium- or potassium-containing alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, andtheir mixtures; (c) Sodium- or potassium-containing oxides, carbides,nitrides, sulfides, phosphides, selenides, tellurides, or antimonides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures orcomposites thereof; (d) sodium or potassium salts; (e) particles ofgraphite, hard carbon, soft carbon or carbon particles and pre-sodiatedversions thereof; and combinations thereof.

The invention also provides a method of extending the cycle-life of analkali metal-sulfur cell. The method comprises implementing twoanode-protecting layers between an anode active material layer and anelectrolyte or porous separator/electrolyte layer. The firstanode-protecting layer has a thickness from 1 nm to 100 μm and comprisesa thin layer (paper, membrane, fabric, etc.) of electron-conductingmaterial selected from graphene sheets, carbon nanotubes, carbonnanofibers, carbon or graphite fibers, expanded graphite flakes, metalnanowires, conductive polymer fibers, or a combination thereof, whereinthe first anode-protecting layer has a specific surface area greaterthan 50 m²/g and is in physical contact with the anode active materiallayer.

The second anode-protecting layer comprises an elastomer having arecoverable tensile elastic strain from 2% to 1,000% (preferably from 5%to 300%), a lithium ion conductivity no less than 10⁻⁸ S/cm (preferably>10⁻⁵ S/cm) at room temperature, and a thickness from 1 nm to 100 μm(preferably from 10 nm to 10 μm). This second anode-protecting layer isdisposed between the first anode protecting layer (covering the lithiummetal or lithium alloy foil or coating) and the porous separator (orsolid-state electrolyte).

In some embodiments, the first anode-protecting layer contains aconductive reinforcement material selected from graphene sheets, carbonnanotubes, carbon nanofibers, carbon or graphite fibers, expandedgraphite flakes, metal nanowires, conductive polymer fibers, or acombination thereof.

The invention also provides a method of manufacturing an alkalimetal-sulfur battery (e.g. a lithium-sulfur battery), the methodcomprising: (a) providing a cathode active material layer and anoptional cathode current collector to support the cathode activematerial layer; (b) providing an anode active material layer (containinga lithium metal or lithium alloy foil or coating) and an optional anodecurrent collector to support the lithium metal or lithium alloy foil orcoating; (c) providing an electrolyte in ionic contact with the anodeactive material layer and the cathode active material layer (i.e.enabling lithium ion or sodium ion transport between the anode and thecathode) and an optional separator electrically separating the anode andthe cathode; (d) providing a first anode-protecting layer having athickness from 1 nm to 100 μm and comprising a thin layer ofelectron-conducting material selected from graphene sheets, carbonnanotubes, carbon nanofibers, carbon or graphite fibers, expandedgraphite flakes, metal nanowires, conductive polymer fibers, or acombination thereof, wherein the first anode-protecting layer has aspecific surface area greater than 50 m²/g and is in physical contactwith the anode active material layer; and (e) providing a secondanode-protecting layer of an elastomer having a recoverable tensileelastic strain from 2% to 1,000% (preferably from 5% to 300%), a lithiumion conductivity no less than 10⁻⁸ S/cm at room temperature, and athickness from 1 nm to 100 μm (preferably from 10 nm to 10 μm). Thissecond anode-protecting layer is disposed between the first anodeprotecting layer (covering the lithium metal or lithium alloy foil orcoating) and the porous separator (or the electrolyte).

In some embodiments, the cathode active material layer comprises asulfur-containing material selected from a sulfur-carbon hybrid,sulfur-graphite hybrid, sulfur-graphene hybrid, conductingpolymer-sulfur hybrid, metal sulfide, sulfur compound, or a combinationthereof. The sulfur-carbon hybrid, sulfur-graphite hybrid,sulfur-graphene hybrid, or conducting polymer-sulfur hybrid may be amixture, blend, composite, chemically or physically bonded entity ofsulfur or sulfide with a carbon, graphite, graphene, or conductingpolymer material.

In the above-defined method, the step of implementing a firstanode-protecting layer may be conducted by spraying a slurry of aconductive material (e.g. graphene sheets and/or CNTs) dispersed in aliquid (e.g. an organic solvent) onto a primary surface of the anodeactive material layer, followed by liquid removal. Alternatively, onemay prepare a layer of such a conductive material (e.g. graphene paper,membrane, CNT fabric, etc.) first, which is then followed by laying thislayer over a primary surface of the anode active material layer (e.g. aLi foil).

The step of implementing a second anode-protecting layer may beconducted by depositing a layer of an elastomer onto one primary surfaceof the first protective layer that in turn covers the anode activematerial layer. This step comprises optionally compressing the protectedanode to improve a contact between the anode-protecting layers and theanode active material layer, followed by combining the protected anode,the separator/electrolyte, and the cathode together to form the lithiummetal secondary battery. A good contact between the anode activematerial layer and the anode-protecting layer is essential to reducinginternal resistance.

In certain embodiments, the step of implementing the anode-protectinglayers is conducted by (i) preparing a conductive material-protectedanode active material layer; (ii) depositing a layer of an elastomeronto one primary surface of the separator to form a coated separator;and (iii) combining the conductive material (first protectinglayer)-protected active anode layer, the coated separator, a cathode,and the electrolyte together to form the lithium metal secondarybattery. A compressive stress may be advantageously applied (e.g. viapress-rolling) to improve the contact between the anode-protecting layerand the anode active material layer to be protected.

In certain embodiments, the step of implementing anode-protecting layersis conducted by forming a first protecting layer of conductive material(e.g. graphene paper sheet, membrane, fabric, etc.) and a secondprotecting layer of elastomer, followed by laminating the anode activematerial layer, the first protecting layer, the second protecting layerof elastomer, the separator layer, the cathode layer, along with theelectrolyte to form the lithium metal secondary battery, wherein anoptional (but desirable) compressive stress is applied to improve thecontact between the anode-protecting layers and the anode activematerial layer during or after this laminating step.

In some preferred embodiments, one may further implement acathode-protecting layer disposed between a cathode active materiallayer and the separator/electrolyte layer. Preferably, thecathode-protecting layer has a thickness from 1 nm to 100 μm andcomprises an elastomer having a fully recoverable tensile elastic strainfrom 2% to 1,000% and a lithium ion conductivity from 10⁻⁸ S/cm to5×10⁻² S/cm when measure at room temperature. The cathode-protectinglayer is implemented mainly for the purpose of reducing or eliminatingthe shuttling effect by preventing S or polysulfide from migrating outof the cathode zone and into the anode zone. More specifically, thislayer acts to block the diffusion of any sulfur or metal polysulfide(e.g. lithium polysulfide or sodium polysulfide) dissolved in thecathode from migrating to the anode side. This effectively reduces oreliminates the shuttling effect. This cathode-protecting layer, beinghighly elastic, also acts to maintain a good contact between theseparator (if liquid or gel electrolyte is used) or the solid-stateelectrolyte and the cathode active material layer. Due to the largevolume expansion/shrinkage of the S cathode, this elastic layer expandsand shrinks congruently or conformably with the cathode active materiallayer, thereby preventing the formation of a gap between the separator(or solid-state electrolyte) and the cathode active material layer.

The anode-protecting layer implemented between the anode active layerand the separator (or the solid-state electrolyte) is mainly for thepurpose of reducing or eliminating the alkali metal dendrite byproviding a more stable alkali metal-electrolyte interface that is moreconducive to uniform deposition of alkali metal during battery charges.This anode-protecting layer also acts to block the penetration of anydendrite, once initiated, from reaching the separator or cathode. Thisanode-protecting layer, being highly elastic, also can shrink or expandsresponsive the thickness increase or decrease of the anode activematerial layer.

It is advantageous to implement both an anode-protecting layer and acathode-protecting layer in the same alkali metal-sulfur cell.

These and other advantages and features of the present invention willbecome more transparent with the description of the following best modepractice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic of a prior art alkali metal-sulfur battery cell;

FIG. 1(B) Schematic of another prior art alkali metal-sulfur batterycell;

FIG. 2 Schematic of the presently invented alkali metal-sulfur cellcontaining two anode-protecting layers and a cathode-protecting layer.Either or two anode protecting layers and, optionally, one or twocathode-protecting layers may be included in a lithium-sulfur cell, Na—Scell, or K—S cell.

FIG. 3 The cycling behaviors of 4 cells, each having a Li foil anode anda cathode containing graphene-supported sulfur particles as the cathodeactive material: one without any protecting layer, one with twoanode-protecting layers but no cathode-protecting layer, one with acathode-protecting layer but no anode-protecting layer, and one with twoanode-protecting layers and one cathode-protecting layer.

FIG. 4 The cathode specific capacity values of 3 Li—S cells; the firstcell has a cathode-protecting layer containing a sulfonated SBS (noanode-protecting layer), the second layer contains 2 anode-protectinglayers (no cathode-protecting layer), and the third cell has nocathode-protecting layer.

FIG. 5 The cathode specific capacity values of three room temperatureNa—S cells each featuring a cathode active material layer containingsulfur-MCMB (activated) composite particles as the cathode activematerial: first cell has a SIBS/RGO composite-based cathode-protectingprotecting layer, second cell has 2 anode-protecting layers and 1cathode protecting layer (SIBS/RGO), and the third cell has t noprotecting layer.

FIG. 6 Ragone plots (cell power density vs. cell energy density) of twoLi metal-sulfur cells: one featuring two anode-protecting layers and theother without a protecting layer. The cathode active material is reducedgraphene oxide-embraced S particles.

FIG. 7 Ragone plots (cell power density vs. cell energy density) of 4alkali metal-sulfur cells each having a cathode active material layercontaining particles of exfoliated graphite worms electrochemicallyimpregnated with sulfur as the cathode active material: one Na—S cellfeaturing two anode-protecting layers, one Na—S cell without aprotecting layer, one K—S cell featuring two anode-protecting layers,and 1 K—S cell without a protecting layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For convenience, the following discussion of preferred embodiments isprimarily based on Li—S cells, but the same or similar composition,structure, and methods are applicable to Na—S and K—S cells. Examplesare presented for Li—S cells, room-temperature Na—S cells, and K—Scells. Conventional Li—S cells are illustrated in FIG. 1(A) and FIG.1(B).

A. Alkali Metal-Sulfur Cells (Using Lithium-Sulfur Cells as an Example)

The specific capacity and specific energy of a Li—S cell (or Na—S, orK—S cell) are dictated by the actual amount of sulfur that can beimplemented in the cathode active layer (relative to other non-activeingredients, such as the binder resin and conductive filler) and theutilization rate of this sulfur amount (i.e. the utilization efficiencyof the cathode active material or the actual proportion of S thatactively participates in storing and releasing lithium ions). Using Li—Scell as an illustrative example, a high-capacity and high-energy Li—Scell requires a high amount of S in the cathode active layer (i.e.relative to the amounts of non-active materials, such as the binderresin, conductive additive, and other modifying or supporting materials)and a high S utilization efficiency). The present invention providessuch a cathode active layer, its constituent powder mass product, theresulting Li—S cell, and a method of producing such a cathode activelayer and battery.

In some embodiments, as illustrated in FIG. 2, the alkali metal-sulfurcell (selected from lithium-sulfur cell, sodium-sulfur cell, orpotassium-sulfur cell, said alkali metal-sulfur cell) comprises: (a) ananode; (b) a cathode active material layer, comprising asulfur-containing material selected from a sulfur-carbon hybrid,sulfur-graphite hybrid, sulfur-graphene hybrid, conductingpolymer-sulfur hybrid, metal sulfide, sulfur compound, or a combinationthereof, and an optional cathode current collector supporting saidcathode active material layer; and (c) an electrolyte orelectrolyte/separator layer; wherein the anode comprises: (i) an anodeactive material layer containing a layer of lithium or lithium alloy, ina form of a foil, coating, or multiple particles aggregated together, asan anode active material and an optional anode current collectorsupporting the anode active material layer; (ii) a firstanode-protecting layer having a thickness from 1 nm to 100 μm andcomprising a thin layer of electron-conducting material selected fromgraphene sheets, carbon nanotubes, carbon nanofibers, carbon or graphitefibers, expanded graphite flakes, metal nanowires, conductive polymerfibers, or a combination thereof, wherein the first anode-protectinglayer has a specific surface area greater than 50 m²/g and is inphysical contact with the anode active material layer; and (iii) asecond anode-protecting layer in physical contact with the firstanode-protecting layer, having a thickness from 1 nm to 100 μm andcomprising an elastomer having a fully recoverable tensile elasticstrain from 2% to 1,000% and a lithium ion conductivity from 10⁻² S/cmto 5×10⁻² S/cm when measure at room temperature.

Preferably, the elastomer layer has a lithium ion conductivity no lessthan 10⁻⁶ S/cm (typically from 10⁻⁵ S/cm to 5×10⁻² S/cm, measured atroom temperature), and a thickness from 10 nm to 20 μm. In someembodiments, the elastomer has from 0.01% to 40% by weight of anadditive dispersed therein.

The first anode-protecting layer, being electron-conducting and having ahigh specific surface area (preferably >50 m²/g) can significantlydecrease the exchange current density imposed on the anode activematerial (the Li metal), to the extent that presumably the localexchange current density can be lower than the threshold exchangecurrent density for lithium dendrite initiation or that for the dendritepropagation, once initiated.

Preferably, the first anode-protecting layer contains a thin membrane,paper, non-woven, woven fabric, etc. of graphene sheets, carbonnanotubes, carbon nanofibers, carbon or graphite fibers, expandedgraphite flakes, metal nanowires, conductive polymer fibers, or acombination thereof. This layer must be reasonably permeable to lithiumions; e.g. having pores to allow for easy migration of lithium ions.

The graphene sheets to be used in the 1^(st) anode-protecting layer orto be dispersed in an elastomer matrix are preferably selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, nitrogenated graphene, hydrogenatedgraphene, doped graphene, functionalized graphene, or a combinationthereof. The graphene sheets preferably comprise single-layer grapheneor few-layer graphene, wherein the few-layer graphene is defined as agraphene platelet formed of less than 10 graphene planes. The carbonnanotubes (CNTs) can be a single-walled CNT or multi-walled CNT. Thecarbon nanofibers may be vapor-grown carbon nanofibers orelectrospinning based carbon nanofibers (e.g. electrospun polymernanofibers that are subsequently carbonized).

Preferably, the elastomer in the 2^(nd) anode-protecting layer containsa sulfonated or non-sulfonated version of an elastomer selected fromnatural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprenerubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrilerubber, ethylene propylene rubber, ethylene propylene diene rubber,metallocene-based poly(ethylene-co-octene) (POE) elastomer,poly(ethylene-co-butene) (PBE) elastomer,styrene-ethylene-butadiene-styrene (SEBS) elastomer, epichlorohydrinrubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber,perfluoroelastomers, polyether block amides, chlorosulfonatedpolyethylene, ethylene-vinyl acetate, thermoplastic elastomer, proteinresilin, protein elastin, ethylene oxide-epichlorohydrin copolymer,polyurethane, urethane-urea copolymer, or a combination thereof.

The sulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphenehybrid, or conducting polymer-sulfur hybrid may be a mixture, blend,composite, or chemically or physically bonded entity of sulfur orsulfide with a carbon, graphite, graphene, or conducting polymermaterial. For instance, a sulfur-graphene hybrid can be a simple mixture(in a particle form) of sulfur and graphene prepared by ball-milling.Such a hybrid can contain sulfur bonded on surfaces of a graphene oxidesheet, etc. As another example, the sulfur-carbon hybrid can be a simplemixture (in a particle form) of sulfur and carbon nanotubes, or cancontain sulfur residing in pores of activated carbon particles. In theinstant cathode layer, these particles of sulfur hybrid are embraced bya sulfonated elastomer composite.

In the rechargeable alkali metal-sulfur cell, the metal sulfide maycontain a material denoted by M_(x)S_(y), wherein x is an integer from 1to 3 and y is an integer from 1 to 10, and M is a metal element selectedfrom an alkali metal, an alkaline metal selected from Mg or Ca, atransition metal, a metal from groups 13 to 17 of the periodic table, ora combination thereof. The metal element M preferably is selected fromLi, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al. In some preferredembodiments, the metal sulfide in the cathode layer contains Li₂S₁,Li₂S₂, Li₂S₃, Li₂S₄, Li₂Ss, Li₂S₆, Li₂S₇, Li₂Ss, Li₂Sg, Li₂S₁₀, Na₂S₁,Na₂S₂, Na₂S₃, Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S₁,K₂S₂, K₂S₃, K₂S₄, K₂S₅, K₂S₆, K₂S₇, K₂S₈, K₂S₉, or K₂S₁₀.

In the rechargeable alkali metal-sulfur cell, the carbon or graphitematerial in the cathode active material layer may be selected frommesophase pitch, mesophase carbon, mesocarbon microbead (MCMB), cokeparticle, expanded graphite flake, artificial graphite particle, naturalgraphite particle, highly oriented pyrolytic graphite, soft carbonparticle, hard carbon particle, carbon nanotube, carbon nanofiber,carbon fiber, graphite nanofiber, graphite fiber, carbonized polymerfiber, activated carbon, carbon black, or a combination thereof. Thegraphene may be selected from pristine graphene, graphene oxide, reducedgraphene oxide (RGO), graphene fluoride, nitrogenated graphene,hydrogenated graphene, doped graphene, functionalized graphene, or acombination thereof.

The conducting polymer-sulfur hybrid may preferably contain anintrinsically conductive polymer selected from polyaniline, polypyrrole,polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivativethereof, or a combination thereof. This can be a simple mixture ofsulfur or metal sulfide with a conducting polymer.

In certain embodiments, the elastomer contains from 0.1% to 40% byweight of a lithium ion-, sodium ion-, or potassium ion-conductingadditive dispersed therein. The lithium ion-conducting additive, alongwith an optional conductive reinforcement material, is dispersed in thesulfonated elastomer matrix and is selected from Li₂CO₃, Li₂O, Li₂C₂O₄,LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S,Li_(x)SO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=ahydrocarbon group, 0≤x≤1, 1≤y≤4.

The lithium ion-conducting additive may be selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethylsulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof. The sodium ion- or potassiumion-conducting additive, dispersed in the UHMW polymer, may be selectedfrom sodium perchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-methanesulfonate (NaCF₃SO₃), potassiumtrifluoro-methanesulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

An elastomer is a high-elasticity material, which exhibits an elasticdeformation that is from at least 2% to 1,000% when measured underuniaxial tension (without an additive or reinforcement in the polymer,but can contain a lithium salt and/or liquid solvent dispersed in thepolymer). In the field of materials science and engineering, the“elastic deformation” is defined as a deformation of a material (whenbeing mechanically stressed) that is essentially fully recoverable andthe recovery is essentially instantaneous upon release of the load. Theelastic deformation of the sulfonated elastomer is preferably andtypically greater than 5%, more preferably and typically greater than10%, further more preferably and typically greater than 30%, still morepreferably greater than 50%, and most preferably greater than 100%.

Not to be bound by theory, but the applicants believe that theanode-protecting layer is capable of performing at least the followingthree functions:

-   -   1) Being highly elastic, the elastomer layer helps to maintain a        good contact between an alkali metal layer (e.g. lithium metal        foil, as the anode active material) and an electrolyte phase        (e.g. an electrolyte or electrolyte/separator assembly) when the        alkali metal layer decreases in thickness (e.g. due to        dissolution of Li in the electrolyte when the battery is        discharged) or increases in thickness (e.g. due to re-deposition        of lithium metal back to the Cu foil or lithium metal when the        battery is recharged). The same function also works for the        alkali metal-sulfur cell containing lithiated Si particles as an        anode active material, as an example. Si particles and the        entire anode active material layer can shrink and expand when        the battery is discharged or charged. The elastomer can expand        and shrink responsive to the shrinkage and expansion of the        anode active material layer. Such a conformal or congruent        expansion/shrinkage of the elastomer helps to eliminate the        potential gap between the anode active material layer and the        electrolyte or separator, thereby maintaining the lithium ion        migration paths required of an operational Li—S battery.    -   2) The elastomer, infiltrated with a liquid electrolyte (before,        during, or after the cell is fabricated) and coupled with its        high-elasticity nature (good electrode-electrolyte contact),        enables the returning alkali metal ions (e.g. L⁺ or Na⁺ ions) to        uniformly and successfully deposit back to the alkali metal        surface, reducing the formation of dead lithium particles (or        dead sodium particles), which otherwise become inactive. The        uniform deposition of alkali metal also reduces the tendency to        form the dangerous Li or Na dendrite.    -   3) The presence of the conductive reinforcement material        (graphene sheets, CNTs, CNFs, etc.) are high strength materials,        capable of stopping or deflecting the growth of dendrites (if        initiated), preventing the dendrite from penetrating the        separator and reaching the cathode side to induce internal        shorting, which otherwise is a fire and explosion hazard.

In certain embodiments, the alkali metal-sulfur cell may furthercomprise one or two cathode protecting layers disposed between thecathode active layer and the separator/electrolyte layer:

-   -   a) a first cathode-protecting layer having a thickness from 1 nm        to 100 μm and comprising a thin layer of electron-conducting        material selected from graphene sheets, carbon nanotubes, carbon        nanofibers, carbon or graphite fibers, expanded graphite flakes,        metal nanowires, conductive polymer fibers, or a combination        thereof, wherein said first anode-protecting layer has a        specific surface area greater than 50 m²/g and is in physical        contact with the cathode active material layer or the        separator/electrolyte layer; and/or b) a second        cathode-protecting layer in physical contact with the first        cathode-protecting layer, having a thickness from 1 nm to 100 μm        and comprising an elastomer having a fully recoverable tensile        elastic strain from 2% to 1,000% and a lithium ion conductivity        from 10⁻⁸ S/cm to 5×10⁻² S/cm when measure at room temperature.

The 1^(st) cathode-protecting layer may be the same as or different thanthe 1^(st) anode-protecting layer. The 2^(nd) cathode-protecting layermay be the same as or different than the 2^(nd) cathode-protectinglayer.

Not to be bound by theory, but the applicants further believe that thecathode-protecting layer is capable of performing at least the followingtwo functions:

-   -   1) Sulfur and lithium polysulfide, and the entire cathode active        material layer, can expand and shrink when the battery is        discharged or charged. The elastomer layer implemented between        the cathode active material layer and the separator layer (or        the electrolyte phase) can shrink and expand responsive to the        expansion and shrinkage of the cathode active material layer.        Such a conformal or congruent expansion/shrinkage of the        sulfonated elastomer composite helps to eliminate the potential        gap between the cathode active material layer and the        electrolyte or separator, thereby maintaining the lithium ion        migration paths required of an operational Li—S battery.    -   2) The conductive material-based layer also acts to trap and        block the sulfur or metal polysulfide species dissolved in the        electrolyte, thereby preventing continuing migration of these        species to the anode side where they react with lithium (or        sodium, or potassium) and are unable to return to the cathode        (the shuttle effect). This shuttle effect is mainly responsible        for continued and rapid capacity decay in a conventional Li—S,        room-temperature Na—S, or K—S cell.

Alternatively, referring to the lower portion of FIG. 3, lithium sulfideis used as the cathode active material. A layer of elastomer mayencapsulate around the lithium polysulfide particle to form a core-shellstructure. When the Li—S battery is charged and lithium ions arereleased from the cathode, the cathode active material particlecontracts. However, the elastomer is capable of elastically shrinking ina conformal manner; hence, leaving behind no gap between the protectiveshell and the sulfur. Such a configuration is amenable to subsequentlithium reaction with sulfur. The elastomer shell expands and shrinkscongruently with the expansion and shrinkage of the encapsulated corecathode active material particle, enabling long-term cycling stabilityof a Li—S or Na—S battery.

B. Sulfonated or Un-Sulfonated Elastomer or Elastomer Composites

Preferably and typically, the elastomer layer has a lithium ionconductivity no less than 10⁻⁶ S/cm, more preferably no less than 10⁻⁴S/cm, further preferably no less than 10⁻³ S/cm, and most preferably noless than 10⁻² S/cm. In some embodiments, the elastomer layer containsno additive or filler dispersed therein. In others, the elastomercontains from 0.1% to 50% (preferably 1% to 35%) by weight of a lithiumion-conducting additive dispersed in an elastomer matrix material. Theelastomer must have a high elasticity (elastic deformation strainvalue>2%). An elastic deformation is a deformation that is fullyrecoverable and the recovery process is essentially instantaneous (nosignificant time delay). The sulfonated elastomer composite can exhibitan elastic deformation from 5% up to 800% (8 times of its originallength), more typically from 10% to 500%, and further more typicallyfrom 30% to 300%.

It may be noted that although a metal or a plastic typically has a highductility (i.e. can be extended to a large strain without breakage), themajority of the deformation is plastic deformation (non-recoverable) andonly a small amount of elastic deformation (typically <1% and moretypically <0.2%). Thus, a metal or a plastic does not qualify as ahigh-elasticity material.

Further, we have unexpectedly discovered that the presence of an amountof a lithium salt or sodium salt (1-35% by weight) and a liquid solvent(0-50%) can significantly increase the lithium-ion or sodium ionconductivity of the sulfonated elastomer matrix.

The first step for producing an elastomer layer is to dissolve anelastomer or its precursor (e.g. uncured oligomer or polymer) in asolvent to form a polymer solution. Subsequently, the conductivereinforcement material and other additive are dispersed in this polymersolution to form a suspension (dispersion or slurry). This suspensioncan then be subjected to a film-forming procedure (e.g. spraying,printing, casting, coating, etc.) and a solvent removal treatment. Theelastomer precursor is then cured or polymerized.

One may dispense and deposit a layer of a first elastomer onto a primarysurface of a protected anode active material layer. Alternatively, onemay dispense and deposit a layer of a first elastomer onto a primarysurface of a separator layer. Further alternatively, one may prepareseparate free-standing discrete layers of the elastomer and othercomponents first. One or both of these layers are then laminatedtogether with the anode layer, separator/electrolyte, and the cathodelayer to form a battery cell.

The procedures of spraying, printing, casting, coating, and laminatingare well-known in the art.

The elastomer may form a mixture or blend with an electron-conductingpolymer selected from polyaniline, polypyrrole, polythiophene,polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonatedversions), or a combination thereof.

In some embodiments, the elastomer may form a mixture with a lithiumion-conducting polymer selected from regular molecular weight (<500,000g/mole) version of poly(ethylene oxide) (PEO), polypropylene oxide(PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA),poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivativethereof (e.g. sulfonated versions), or a combination thereof.

A wide variety of rubbers or elastomers may be readily sulfonated usingknown sulfonation procedures. Unsaturated rubbers that can be vulcanizedto become elastomer include natural polyisoprene (e.g.cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprenegutta-percha), synthetic polyisoprene (IR for isoprene rubber),polybutadiene (BR for butadiene rubber), chloroprene rubber (CR),polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymerof isobutylene and isoprene, IIR), including halogenated butyl rubbers(chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR),styrene-butadiene rubber (copolymer of styrene and butadiene, SBR),nitrile rubber (copolymer of butadiene and acrylonitrile, NBR),

Some elastomers are saturated rubbers that cannot be cured by sulfurvulcanization; they are made into a rubbery or elastomeric material viadifferent means: e.g. by having a copolymer domain that holds otherlinear chains together. Each of these elastomers can be used toencapsulate particles of an anode active material by one of severalmeans: melt mixing (followed by pelletizing and ball-milling, forinstance), solution mixing (dissolving the anode active materialparticles in an uncured polymer, monomer, or oligomer, with or withoutan organic solvent) followed by drying (e.g. spray drying), interfacialpolymerization, or in situ polymerization of elastomer in the presenceof anode active material particles.

Sulfonated saturated rubbers and related elastomers in this categoryinclude EPM (ethylene propylene rubber, a copolymer of ethylene andpropylene), EPDM rubber (ethylene propylene diene rubber, a terpolymerof ethylene, propylene and a diene-component), epichlorohydrin rubber(ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ),fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such asViton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM:Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA),chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinylacetate (EVA), thermoplastic elastomers (TPE), protein resilin, andprotein elastin. Sulfonated polyurethane and its copolymers (e.g.urea-urethane copolymer) are particularly useful elastomeric shellmaterials for encapsulating anode active material particles.

A variety of synthetic methods may be used to sulfonate an elastomer orrubber: (i) exposure to sulfur trioxide in vapor phase or in solution,possibly in presence of Lewis bases such as triethyl phosphate,tetrahydrofuran, dioxane, or amines; (ii) chlorosulfonic acid in diethylether; (iii) concentrated sulfuric acid or mixtures of sulfuric acidwith alkyl hypochlorite; (iv) bisulfites combined to dioxygen, hydrogenperoxide, metallic catalysts, or peroxo derivates; and (v) acetylsulfate.

Sulfonation of an elastomer or rubber may be conducted before, during,or after curing of the elastomer or rubber. Further, sulfonation of theelastomer or rubber may be conducted before or after the particles of anelectrode active material are embraced or encapsulated by theelastomer/rubber or its precursor (monomer or oligomer). Sulfonation ofan elastomer or rubber may be accomplished by exposing theelastomer/rubber to a sulfonation agent in a solution state or meltstate, in a batch manner or in a continuous process. The sulfonatingagent may be selected from sulfuric acid, sulfonic acid, sulfurtrioxide, chlorosulfonic acid, a bisulfate, a sulfate (e.g. zincsulfate, acetyl sulfate, etc.), a mixture thereof, or a mixture thereofwith another chemical species (e.g. acetic anhydride, thiolacetic acid,or other types of acids, etc.). In addition to zinc sulfate, there are awide variety of metal sulfates that may be used as a sulfonating agent;e.g. those sulfates containing Mg, Ca, Co, Li, Ba, Na, Pb, Ni, Fe, Mn,K, Hg, Cr, and other transition metals, etc.

For instance, a triblock copolymer, poly(styrene-isobutylene-styrene) orSIBS, may be sulfonated to several different levels ranging from 0.36 to2.04 mequiv./g (13 to 82 mol % of styrene; styrene being 19 mol % of theunsulfonated block copolymer). Sulfonation of SIBS may be performed insolution with acetyl sulfate as the sulfonating agent. First, aceticanhydride reacts with sulfuric acid to form acetyl sulfate (asulfonating agent) and acetic acid (a by-product). Then, excess water isremoved since anhydrous conditions are required for sulfonation of SIBS.The SIBS is then mixed with the mixture of acetyl sulfate and aceticacid. Such a sulfonation reaction produces sulfonic acid substituted tothe para-position of the aromatic ring in the styrene block of thepolymer. Elastomers having an aromatic ring may be sulfonated in asimilar manner.

A sulfonated elastomer also may be synthesized by copolymerization of alow level of functionalized (i.e. sulfonated) monomer with anunsaturated monomer (e.g. olefinic monomer, isoprene monomer oroligomer, butadiene monomer or oligomer, etc.).

C. Additional Details about the Structure of Li—S, Na—S, and K—S Cells

The anode active material layer of an alkali metal-sulfur cell cancontain a foil or coating of Li, Na, or K supported by a currentcollector (e.g. Cu foil), as illustrated in the left-hand portion ofFIG. 1(A) for a prior art Li—S cell. Alternatively, the anode activematerial may contain, for instance, particles of prelithiated Siparticles or surface-stabilized Li particles, as illustrated in FIG. 2.However, the cathode layer in the instant cell is distinct, as alreadydiscussed above.

The electrolyte for an alkali metal-sulfur cell may be an organicelectrolyte, ionic liquid electrolyte, gel polymer electrolyte,solid-state electrolyte (e.g. polymer solid electrolyte or inorganicsolid electrolyte), quasi-solid electrolyte or a combination thereof.The electrolyte typically contains an alkali metal salt (lithium salt,sodium salt, and/or potassium salt) dissolved in a solvent.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, a roomtemperature ionic liquid solvent, or a combination thereof.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂], lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt, sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-methanesulfonate (NaCF₃SO₃), potassiumtrifluoro-methanesulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI), andbis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂). Among them,LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ are preferred for Li—S cells, NaPF₆ andLiBF₄ for Na—S cells, and KBF₄ for K—S cells. The content ofaforementioned electrolytic salts in the non-aqueous solvent ispreferably 0.5 to 3.0 M (mol/L) at the cathode side and 3.0 to >10 M atthe anode side.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulfonamide (TFSI) anion.

This combination gives a fluid with an ionic conductivity comparable tomany organic electrolyte solutions and a low decomposition propensityand low vapor pressure up to ˜300-400° C. This implies a generally lowvolatility and non-flammability and, hence, a much safer electrolyte forbatteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulfonyl) imide, bis(fluorosulfonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂FsBF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a Li—S cell.

In the presently invented alkali metal-sulfur cell, the cathode activelayer comprises sulfur or a sulfur-containing compound preferablysupported by or embedded in a conducting material, forming asulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid,conducting polymer-sulfur hybrid, metal sulfide, sulfur compound, etc.These hybrid or compound materials are produced in the form of particlesthat contain a mixture, blend, composite, or bonded entity of sulfur orsulfide with a carbon, graphite, graphene, or conducting polymermaterial. Metal sulfides (e.g. lithium polysulfide, sodium polysulfide,etc.) and sulfur compounds are readily available in a fine particleform. Sulfur can be combined with a conducting material (carbon,graphite, graphene, and/or conducting polymer) to form a composite,mixture, or bonded entity (e.g. sulfur bonded on graphene oxidesurface).

There are many well-known procedures that can be used to make theaforementioned sulfur-containing materials into particles. For instance,one may mix solid sulfur with a carbon or graphite material to formcomposite particles using ball-milling. The resulting particles aretypically ellipsoidal or potato-like in shape having a size from 1 to 20μm. Also, one may infiltrate S or sulfide into the pores of porouscarbon or graphite particles (e.g. activated carbon, mesoporous carbon,activated carbon fibers, etc.) using vapor phase infiltration, solutioninfiltration, chemical infiltration, or electrochemical infiltration.Alternatively, one may deposit sulfur onto surfaces of graphene sheets,CNTs, carbon nanofibers, etc. and then form these S-coated nanomaterialsinto a spherical or ellipsoidal shape using high-intensity ball-milling,spray-drying (of their suspensions), aerosol formation, etc. Theseparticles are then encapsulated with a sulfonated elastomer compositeusing the micro-encapsulation processes discussed above. The cathode ina conventional Li—S cell typically has less than 70% by weight of sulfurin a composite cathode composed of sulfur and the conductiveadditive/support. Even when the sulfur content in the prior artcomposite cathode reaches or exceeds 70% by weight, the specificcapacity of the composite cathode is typically significantly lower thanwhat is expected based on theoretical predictions. For instance, thetheoretical specific capacity of sulfur is 1,675 mAh/g. A compositecathode composed of 70% sulfur (S) and 30% carbon black (CB), withoutany binder, should be capable of storing up to 1,675×70%=1,172 mAh/g.Unfortunately, the observed specific capacity is typically less than 75%or 879 mAh/g (often less than 50% or 586 mAh/g in this example) of whatcould be achieved. In other words, the active material (S) utilizationrate is typically less than 75% (or even<50%). This has been a majorissue in the art of Li—S cells and there has been no solution to thisproblem.

Thus, it is highly advantageous to obtain a high sulfur loading and yet,concurrently, maintaining an ultra-thin/small thickness/diameter ofsulfur for significantly enhanced sulfur utilization efficiency, energydensity and power density. For instance, one can deposit nanoscaledsulfur (1-5 nm thick) on graphene surfaces using chemical,electrochemical, or vapor deposition to form S-coated or S-bondedgraphene sheets. These S-coated or S-bonded graphene sheets are thenaggregated together using a tumbling mixing, ball-milling, or sprayingprocedure. These steps enable the preparation of S-conducting materialhybrids that contain 85%-99% by weight sulfur, yet maintaining a coatingthickness or particle diameter from 1 nm to 5 nm. This ultra-smalldimension enables fast lithium diffusion and lithium-sulfur reactions,leading to high S utilization efficiency (hence, high energy density)even at high charge-discharge rates. Several procedures of producingsuch small S particles or coating are illustrated in examples of thisspecification.

Again, the shuttling effect is related to the tendency for sulfur oralkali metal polysulfide that forms at the cathode to get dissolved inthe solvent and for the dissolved lithium polysulfide species to migratefrom the cathode to the anode, where they irreversibly react withlithium to form species that prevent sulfide from returning back to thecathode during the subsequent discharge operation of the Li—S cell (thedetrimental shuttling effect). It appears that by implementing acathode-protecting layer of a sulfonated elastomer composite, we havesignificantly reduced and even eliminated the shuttling effect,resulting in an alkali metal battery that has an exceptionally longcycle-life. We have solved the most critical, long-standing problem ofalkali metal-sulfur batteries.

In all versions of the above-discussed alkali metal-sulfur cells, theanode active material may contain, as an example, lithium metal foil (orpowder) or a high-capacity Si, Sn, Al, or SnO₂ capable of storing agreat amount of lithium. The cathode active material may contain puresulfur (if the anode active material contains lithium), lithiumpolysulfide, or any sulfur containing compound, molecule, or polymer. Ifthe cathode active material includes lithium-containing species (e.g.lithium polysulfide) when the cell is made, the anode active materialcan be any material capable of storing a large amount of lithium (e.g.Si, Ge, Sn, Al, SnO₂, etc.).

At the anode side, when lithium metal is used as the sole anode activematerial in a Li—S cell, there is concern about the formation of lithiumdendrites, which could lead to internal shorting and thermal runaway.Herein, we have used two approaches, separately or in combination, toaddress this dendrite formation issue: one involving the use of aprotecting layer containing a sulfonated elastomer composite discussedabove and the other the use of a nanostructure composed of conductivenanofilaments as an extended anode current collector. For the latter,multiple conductive nanofilaments are processed to form an integratedaggregate structure, preferably in the form of a closely packed web,mat, or paper, characterized in that these filaments are intersected,overlapped, or somehow bonded (e.g., using a binder material) to oneanother to form a network of electron-conducting paths. The integratedstructure has substantially interconnected pores to accommodateelectrolyte. The nanofilament may be selected from, as examples, acarbon nanofiber (CNF), graphite nanofiber (GNF), carbon nanotube (CNT),metal nanowire (MNW), conductive nanofibers obtained by electrospinning,conductive electrospun composite nanofibers, nanoscaled grapheneplatelet (NGP), or a combination thereof. The nanofilaments may bebonded by a binder material selected from a polymer, coal tar pitch,petroleum pitch, mesophase pitch, coke, or a derivative thereof.

Nano fibers may be selected from the group consisting of an electricallyconductive electrospun polymer fiber, electrospun polymer nanocompositefiber comprising a conductive filler, nano carbon fiber obtained fromcarbonization of an electrospun polymer fiber, electrospun pitch fiber,and combinations thereof. For instance, a nanostructured electrode canbe obtained by electrospinning of polyacrylonitrile (PAN) into polymernanofibers, followed by carbonization of PAN. It may be noted that someof the pores in the structure, as carbonized, are greater than 100 nmand some smaller than 100 nm.

The presently invented protective layers may be incorporated in one ofat least four broad classes of rechargeable alkali metal-sulfur cells(using Li as an example):

-   -   (A) Lithium metal-sulfur with a conventional anode        configuration: The cell contains an optional cathode current        collector, a cathode layer, a separator/electrolyte, an anode,        and an anode current collector. Potential dendrite formation may        be overcome by using the invented anode-protecting layer. There        can be a cathode-protecting layer.    -   (B) Lithium metal-sulfur cell with a nanostructured anode        configuration: The cell contains an optional cathode current        collector, a cathode, a separator/electrolyte, an optional anode        current collector, and a nanostructure to accommodate lithium        metal that is deposited back to the anode during a charge or        re-charge operation. This nanostructure (web, mat, or paper) of        nanofilaments provide a uniform electric field enabling uniform        Li metal deposition, reducing the propensity to form dendrites.        This configuration can provide a dendrite-free cell for a long        and safe cycling behavior. Additionally, there can be an        anode-protecting layer and/or a cathode-protecting layer.    -   (C) Lithium ion-sulfur cell with a conventional anode: For        instance, the cell contains an anode composed of anode active        graphite particles bonded by a binder, such as polyvinylidene        fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also        contains a cathode current collector, a cathode, a        separator/electrolyte, and an anode current collector. There can        be an anode-protecting layer and/or a cathode-protecting layer;        and    -   (D)Lithium ion-sulfur cell with a nanostructured anode: For        instance, the cell contains a web of nanofibers coated with Si        coating or bonded with Si nanoparticles. The cell also contains        an optional cathode current collector, a cathode, a        separator/electrolyte, and an anode current collector. There can        be an anode-protecting layer and/or a cathode-protecting layer.        This configuration provides an ultra-high capacity, high energy        density, and a safe and long cycle life.

In the lithium-ion sulfur cell (e.g. as described in (C) and (D) above),the anode active material can be selected from a wide range ofhigh-capacity materials, including (a) silicon (Si), germanium (Ge), tin(Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al),nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), andcadmium (Cd), and lithiated versions thereof; (b) alloys orintermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd withother elements, and lithiated versions thereof, wherein said alloys orcompounds are stoichiometric or non-stoichiometric; (c) oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites, and lithiated versions thereof; (d) salts andhydroxides of Sn and lithiated versions thereof; (e) carbon or graphitematerials and prelithiated versions thereof; and combinations thereof.Non-lithiated versions may be used if the cathode side contains lithiumpolysulfides or other lithium sources when the cell is made.

A possible lithium metal cell may be comprised of an anode currentcollector, an electrolyte phase (optionally but preferably supported bya porous separator, such as a porous polyethylene-polypropyleneco-polymer film), a cathode of the instant invention, and an optionalcathode collector.

For a sodium ion-sulfur cell or potassium ion-sulfur cell, the anodeactive material layer can contain an anode active material selected fromthe group consisting of: (a) Sodium- or potassium-doped silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc(Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni), manganese(Mn), cadmium (Cd), and mixtures thereof; (b) Sodium- orpotassium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c) Sodium-or potassium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof;(d) Sodium or potassium salts; (e) particles of graphite, hard carbon,soft carbon or carbon particles and pre-sodiated versions thereof(pre-doped or pre-loaded with Na), and combinations thereof.

Example 1: Mixing of Sulfur with Carbon/Graphite Particles ViaBall-Milling to Form Sulfur-Containing Particles

Sulfur and lithium polysulfide particles and particles of soft carbon(i.e. graphitizable disordered carbon), natural graphite, mesophasecarbon, expanded graphite flakes, carbon nanofibers, and graphene sheets(50% to 85% by weight of S in the resulting composite or hybrid) werephysically blended and then subjected to ball milling for 2-24 hours toobtain S-containing composite particles (typically in a ball or potatoshape). The particles have a typical size of 1-10 μm. These particles,along with a conductive additive (5% by wt.) and a resin binder (PVDF,5%), were then combined and made into a layer of cathode using thewell-known slurry coating procedure.

Example 2: Simple Sulfur Melt or Liquid Solution Mixing

One way to combine sulfur with a conducting material (e.g.carbon/graphite particles) is to use a solution or melt mixing process.Highly porous activated carbon particles, chemically etched mesocarbonmicrobeads (activated MCMBs), and exfoliated graphite worms were mixedwith sulfur melt at 117-120° C. (slightly above the melting point of S,115.2° C.) for 10-60 minutes to obtain sulfur-impregnated carbonparticles for use a cathode active material.

Example 3: Preparation of Sulfur-Coated Graphene Sheets and TheirSecondary Particles (Particulates)

The step involves producing vapor of elemental sulfur, allowingdeposition of S vapor on surfaces of single-layer or few-layer graphenesheets. The graphene sheets, suspended in a liquid medium (e.g. grapheneoxide in water or graphene in NMP), were sprayed onto a substrate (e.g.glass surface) to form a thin layer of graphene sheets. This thin layerof graphene was then exposed to sublimation-generated physical vapordeposition. Sublimation of solid sulfur occurs at a temperature greaterthan 40° C., but a significant and practically useful sublimation ratetypically does not occur until the temperature is above 100° C. Wetypically used 117-160° C. with a vapor deposition time of 10-120minutes to deposit a thin film of sulfur on graphene surface (sulfurthickness being approximately from 1 nm to 10 nm). This thin layer ofgraphene having a thin film of sulfur deposited thereon was then easilybroken into pieces of S-coated graphene sheets using an air jet mill.Some of these sheets were made into secondary particles of approximately5-15 μm in diameter (e.g. via spray-drying) and used as a cathode activematerial.

Example 4: Electrochemical Impregnation of S in Various PorousCarbon/Graphite Particles

The electrochemical impregnation of S into pores of activated carbonfibers, activated carbon nanotubes, and activated artificial graphiteparticles was conducted by aggregating these particles/fibers into aloosely packed layer. In this approach, an anode, electrolyte, and alayer of such a loosely packed structure (serving as a cathode layer)are positioned in an external container outside of a lithium-sulfurcell. The needed apparatus is similar to an electro-plating system,which is well-known in the art.

In a typical procedure, a metal polysulfide (M_(x)S_(y)) was dissolvedin a solvent (e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1)to form an electrolyte solution. An amount of a lithium salt may beoptionally added, but this is not required for external electrochemicaldeposition. A wide variety of solvents can be utilized for this purposeand there is no theoretical limit to what type of solvents can be used;any solvent can be used provided that there is some solubility of themetal polysulfide in this desired solvent. A greater solubility wouldmean a larger amount of sulfur can be derived from the electrolytesolution.

The electrolyte solution was then poured into a chamber or reactor undera dry and controlled atmosphere condition (e.g. He or nitrogen gas). Ametal foil was used as the anode and a layer of the porous structure asthe cathode; both being immersed in the electrolyte solution. Thisconfiguration constitutes an electrochemical impregnation and depositionsystem. The step of electrochemically impregnating sulfur into pores wasconducted at a current density in the range from 1 mA/g to 10 A/g, basedon the layer weight of the porous carbon/graphite particles/fibers.

The chemical reactions that occur in this reactor may be represented bythe following equation: M_(x)S_(y)→M_(x)S_(y-z)+zS (typically z=1-4).The sulfur coating thickness or particle diameter and the amount of Scoating/particles impregnated may be controlled by the electro-chemicalreaction current density, temperature and time. In general, a lowercurrent density and lower reaction temperature lead to a more uniformimpregnation of S and the reactions are easier to control. A longerreaction time leads to a larger amount of S saturated in the pores.Additionally, the electrochemical method is capable of rapidlyconverting the impregnated S into metal polysulfide (lithiumpolysulfide, sodium polysulfide, and potassium polysulfide, etc.).

Example 5: Chemical Reaction-Induced Impregnation of Sulfur

A chemical impregnation method was herein utilized to prepareS-impregnated carbon fibers that have been chemically activated. Theprocedure began with adding 0.58 g Na₂S into a flask that had beenfilled with 25 ml distilled water to form a Na₂S solution. Then, 0.72 gelemental S was suspended in the Na₂S solution and stirred with amagnetic stirrer for about 2 hours at room temperature. The color of thesolution changed slowly to orange-yellow as the sulfur dissolved. Afterdissolution of the sulfur, a sodium polysulfide (Na₂S_(x)) solution wasobtained (x=4-10).

Subsequently, a sulfur-impregnated carbon fiber sample was prepared by achemical impregnation method in an aqueous solution. First, 180 mg ofexpansion-treated carbon fibers was suspended in 180 ml ultrapure waterwith a surfactant and then sonicated at 50° C. for 5 hours to form astable carbon fiber dispersion. Subsequently, the Na₂S solution wasadded to the above-prepared dispersions in the presence of 5 wt %surfactant cetyl trimethyl-ammonium bromide (CTAB), the as-preparedcarbon fiber/Na₂S, blended solution was sonicated for another 2 hoursand then titrated into 100 ml of 2 mol/L HCOOH solution at a rate of30-40 drops/min and stirred for 2 hours. Finally, the precipitate wasfiltered and washed with acetone and distilled water several times toeliminate salts and impurities. After filtration, the precipitate wasdried at 50° C. in a drying oven for 48 hours. The reaction may berepresented by the following reaction: S_(x) ²⁻+2H⁺→(x−1) S+H₂S.

Example 6: Redox Chemical Reaction-Induced Impregnation of Sulfur inActivated MCMBs and Activated Needle Coke

In this chemical reaction-based deposition process, sodium thiosulfate(Na₂S₂O₃) was used as a sulfur source and HCl as a reactant. Anactivated MCMB-water or activated needle coke-water suspension wasprepared and then the two reactants (HCl and Na₂S₂O₃) were poured intothis suspension. The reaction was allowed to proceed at 25-75° C. for1-3 hours, leading to impregnation of S into pores of the activatedstructures. The reaction may be represented by the following reaction:2HCl+Na₂S₂O₃→2NaCl+S↓+SO₂↑+H₂O.

Both non-sulfonated and sulfonated elastomers are used to build thesecond anode-protecting layer in the present invention. The sulfonatedversions typically provide a much higher lithium ion conductivity and,hence, enable higher-rate capability or higher power density. Theelastomer matrix can contain a lithium ion-conducting additive and/or,an electron-conducting additive. Examples 7-10 below provides somerepresentative procedures on preparation of sulfonated elastomers andsulfonated elastomer composites:

Example 7: Sulfonation of Triblock CopolymerPoly(styrene-isobutylene-styrene) or SIBS

An example of the sulfonation procedure used in this study is summarizedas follows: a 10% (w/v) solution of SIBS (50 g) and a desired amount ofgraphene oxide sheets (0.15% to 40.5% by wt.) in methylene chloride (500ml) was prepared. The solution was stirred and refluxed at approximately40 8C, while a specified amount of acetyl sulfate in methylene chloridewas slowly added to begin the sulfonation reaction. Acetyl sulfate inmethylene chloride was prepared prior to this reaction by cooling 150 mlof methylene chloride in an ice bath for approximately 10 min. Aspecified amount of acetic anhydride and sulfuric acid was then added tothe chilled methylene chloride under stirring conditions. Sulfuric acidwas added approximately 10 min after the addition of acetic anhydridewith acetic anhydride in excess of a 1:1 mole ratio. This solution wasthen allowed to return to room temperature before addition to thereaction vessel.

After approximately 5 h, the reaction was terminated by slowly adding100 ml of methanol. The reacted polymer solution was then precipitatedwith deionized water. The precipitate was washed several times withwater and methanol, separately, and then dried in a vacuum oven at 50°C. for 24 h. This washing and drying procedure was repeated until the pHof the wash water was neutral. After this process, the final polymeryield was approximately 98% on average. This sulfonation procedure wasrepeated with different amounts of acetyl sulfate to produce severalsulfonated polymers with various levels of sulfonation or ion-exchangecapacities (IECs). The mol % sulfonation is defined as: mol %=(moles ofsulfonic acid/moles of styrene)×100%, and the IEC is defined as themille-equivalents of sulfonic acid per gram of polymer (mequiv./g).

After sulfonation and washing of each polymer, the S-SIBS samples weredissolved in a mixed solvent of toluene/hexanol (85/15, w/w) withconcentrations ranging from 0.5 to 2.5% (w/v). Desired amounts ofadditives (if not added at an earlier stage) were then added into thesolution to form slurry samples. The slurry samples were slot-die coatedinto layers of sulfonated elastomer for use as an anode-protecting layeror a cathode-protecting layer. A Li—S or Na—S cell can have both theanode-protecting layers (including both the 1^(st) anode-protectinglayer and the 2^(nd) anode-protecting layer) and one or twocathode-protecting layers.

Example 8: Synthesis of Sulfonated Polybutadiene (PB) by Free RadicalAddition of Thiolacetic Acid (TAA) Followed by In Situ Oxidation withPerformic Acid

A representative procedure is given as follows. PB (8.0 g) was dissolvedin toluene (800 mL) under vigorous stirring for 72 h at room temperaturein a 1 L round-bottom flask. Benzophenone (BZP) (0.225 g; 1.23 mmol;BZP/olefin molar ratio=1:120) and TAA (11.9 mL; 0.163 mol, TAA/olefinmolar ratio=1.1) and a desired amount of fillers (0.1%-40% by wt.) wereintroduced into the reactor, and the polymer solution was irradiated for1 h at room temperature with UV light of 365 nm and power of 100 W.

The resulting thioacetylated polybutadiene (PB-TA) was isolated bypouring 200 mL of the toluene solution in a plenty of methanol and thepolymer recovered by filtration, washed with fresh methanol, and driedin vacuum at room temperature (Yield=3.54 g). Formic acid (117 mL; 3.06mol; HCOOH/olefin molar ratio=25) were added to the toluene solution ofPB-TA at 50° C. followed by slow addition of 52.6 mL of hydrogenperoxide (35 wt %; 0.61 mol; H₂O₂/olefin molar ratio=5) in 20 min. Wewould like to caution that the reaction is autocatalytic and stronglyexothermic. The resulting slurry was coated onto a PET film to obtainwet sulfonated polybutadiene (PB-SA) layers, which were then dried toproduce the desired electrode-protecting layers.

It may be noted that the electron-conducting and/or ion-conductingfillers may be added at different stages of the procedure: before,during or after BZP is added or before/during/after the active materialparticles are added.

Example 9: Synthesis of Sulfonated SBS

Sulfonated styrene-butadiene-styrene triblock copolymer (SBS) basedelastomer was directly synthesized. First, SBS (optionally along withsome fillers) is first epoxidized by performic acid formed in situ,followed by ring-opening reaction with an aqueous solution of NaHSO₃. Ina typical procedure, epoxidation of SBS was carried out via reaction ofSBS in cyclohexane solution (SBS concentration=11 g/100 mL) withperformic acid formed in situ from HCOOH and 30% aqueous H₂O₂ solutionat 70° C. for 4 h, using 1 wt % poly(ethylene glycol)/SBS as a phasetransfer catalyst. The molar ratio of H₂O₂/HCOOH was 1. The product(ESBS) was precipitated and washed several times with ethanol, followedby drying in a vacuum dryer at 60° C.

Subsequently, ESBS was first dissolved in toluene to form a solutionwith a concentration of 10 g/100 mL, into which was added 5 wt %TEAB/ESBS as a phase transfer catalyst and 5 wt % DMA/ESBS as aring-opening catalyst. Herein, TEAB=tetraethyl ammonium bromide andDMA=N,N-dimethyl aniline. An aqueous solution of NaHSO₃ and Na₂SO₃(optionally along with graphene sheets or CNTs, if not added earlier)was then added with vigorous stirring at 60° C. for 7 h at a molar ratioof NaHSO₃/epoxy group at 1.8 and a weight ratio of Na₂SO₃/NaHSO₃ at 36%.This reaction allows for opening of the epoxide ring and attaching ofthe sulfonate group according to the following reaction:

The reaction was terminated by adding a small amount of acetone solutioncontaining antioxidant. The mixture was washed with distilled water andthen precipitated by ethanol while being cast into thin films, followedby drying in a vacuum dryer at 50° C. It may be noted that graphenesheets (or CNTs, etc.) and/or ion-conducting fillers may be added duringvarious stages of the aforementioned procedure (e.g. right from thebeginning, or prior to the ring opening reaction).

Example 10: Synthesis of Sulfonated SBS by Free Radical Addition ofThiolacetic Acid (TAA) Followed by in Situ Oxidation with Performic Acid

A representative procedure is given as follows. SBS (8.000 g) in toluene(800 mL) was left under vigorous stirring for 72 hours at roomtemperature and heated later on for 1 h at 65° C. in a 1 L round-bottomflask until the complete dissolution of the polymer. Thus, benzophenone(BZP, 0.173 g; 0.950 mmol; BZP/olefin molar ratio=1:132) and TAA (8.02mL; 0.114 mol, TAA/olefin molar ratio=1.1) were added, and the polymersolution was irradiated for 4 h at room temperature with UV light of 365nm and power of 100 W. To isolate a fraction of the thioacetylatedsample (S(B-TA)S), 20 mL of the polymer solution was treated with plentyof methanol, and the polymer was recovered by filtration, washed withfresh methanol, and dried in vacuum at room temperature. The toluenesolution containing the thioacetylated polymer was equilibrated at 50°C., and 107.4 mL of formic acid (2.84 mol; HCOOH/olefin molarratio=27.5) and 48.9 mL of hydrogen peroxide (35 wt %; 0.57 mol;H₂O₂/olefin molar ratio=5.5) were added in about 15 min. It may becautioned that the reaction is autocatalytic and strongly exothermic!The conductive reinforcement material was added before or after thisreaction. The resulting slurry was stirred for 1 h, and then most of thesolvent was distilled off in vacuum at 35° C. Finally, the slurrycontaining the sulfonated elastomer was added with acetonitrile, castinto films, washed with fresh acetonitrile, and dried in vacuum at 35°C. to obtain layers of sulfonated elastomers.

Other elastomers (e.g. polyisoprene, EPDM, EPR, polyurethane, etc.) weresulfonated in a similar manner. Alternatively, all the rubbers orelastomers can be directly immersed in a solution of sulfuric acid, amixture of sulfuric acid and acetyl sulfate, or other sulfonating agentdiscussed above to produce sulfonated elastomers/rubbers. Again, desiredamounts of fillers may be added at various stages of the procedure.

Example 11: Graphene Oxide From Sulfuric Acid Intercalation andExfoliation of MCMBs

MCMB (mesocarbon microbeads) were supplied by China Steel Chemical Co.This material has a density of about 2.24 g/cm³ with a median particlesize of about 16 Aim. MCMBs (10 grams) were intercalated with an acidsolution (sulfuric acid, nitric acid, and potassium permanganate at aratio of 4:1:0.05) for 48 hours. Upon completion of the reaction, themixture was poured into deionized water and filtered. The intercalatedMCMBs were repeatedly washed in a 5% solution of HCl to remove most ofthe sulfate ions. The sample was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The slurry was dried andstored in a vacuum oven at 60° C. for 24 hours. The dried powder samplewas placed in a quartz tube and inserted into a horizontal tube furnacepre-set at a desired temperature, 800° C.-1,100° C. for 30-90 seconds toobtain graphene samples. A small quantity of graphene was mixed with 5water and ultrasonicated at 60-W power for 10 minutes to obtain asuspension. A small amount was sampled out, dried, and investigated withTEM, which indicated that most of the NGPs were between 1 and 10 layers.The oxygen content of the graphene powders (GO or RGO) produced was from0.1% to approximately 25%, depending upon the exfoliation temperatureand time.

Described below (Examples 12-15) are some representative processes forproducing several types of graphene sheets. These graphene sheets can bereadily made into graphene paper, fabric, or foam layers usingprocedures well-known in the art.

Example 12: Oxidation and Exfoliation of Natural Graphite

Graphite oxide was prepared by oxidation of graphite flakes withsulfuric acid, sodium nitrate, and potassium permanganate at a ratio of4:1:0.05 at 30° C. for 48 hours, according to the method of Hummers[U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of thereaction, the mixture was poured into deionized water and filtered. Thesample was then washed with 5% HCl solution to remove most of thesulfate ions and residual salt and then repeatedly rinsed with deionizedwater until the pH of the filtrate was approximately 4. The intent wasto remove all sulfuric and nitric acid residue out of graphiteinterstices. The slurry was dried and stored in a vacuum oven at 60° C.for 24 hours.

The dried, intercalated (oxidized) compound was exfoliated by placingthe sample in a quartz tube that was inserted into a horizontal tubefurnace pre-set at 1,050° C. to obtain highly exfoliated graphite. Theexfoliated graphite was dispersed in water along with a 1% surfactant at45° C. in a flat-bottomed flask and the resulting graphene oxide (GO)suspension was subjected to ultrasonication for a period of 15 minutesto obtain a homogeneous graphene-water suspension.

Example 13: Preparation of Pristine Graphene Sheets

Pristine graphene sheets were produced by using the directultrasonication or liquid-phase exfoliation process. In a typicalprocedure, five grams of graphite flakes, ground to approximately 20 μmin sizes, were dispersed in 1,000 mL of deionized water (containing 0.1%by weight of a dispersing agent, Zonyl® FSO from DuPont) to obtain asuspension. An ultrasonic energy level of 85 W (Branson S450Ultrasonicator) was used for exfoliation, separation, and size reductionof graphene sheets for a period of 15 minutes to 2 hours. The resultinggraphene sheets were pristine graphene that had never been oxidized andwere oxygen-free and relatively defect-free. There are substantially noother non-carbon elements.

Example 14: Preparation of Graphene Fluoride (GF) Sheets

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). A pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, andthen the reactor was closed and cooled to liquid nitrogen temperature.Subsequently, no more than 1 g of HEG was put in a container with holesfor ClF₃ gas to access the reactor. After 7-10 days, a gray-beigeproduct with approximate formula C₂F was formed. GF sheets were thendispersed in halogenated solvents to form suspensions.

Example 15: Preparation of Nitrogenated Graphene Sheets

Graphene oxide (GO), synthesized in Example 2, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s.

The product was washed several times with deionized water and vacuumdried. In this method graphene oxide gets simultaneously reduced anddoped with nitrogen. The products obtained with graphene/urea massratios of 1/0.5, 1/1 and 1/2 are designated as N-1, N-2 and N-3respectively and the nitrogen contents of these samples were 14.7, 18.2and 17.5 wt. % respectively as determined by elemental analysis. Thesenitrogenated graphene sheets remain dispersible in water.

Example 16: Elastomer and Sulfonated Elastomer Composite-BasedProtective Layers

Several tensile testing specimens were cut from sulfonated elastomerfilms (with or without additive/reinforcement) and tested with auniversal testing machine. The testing results indicate that this seriesof elastomers have an elastic deformation from approximately 160% to760%. The above are for neat sulfonated elastomers without any additive.The addition of up to 30% by weight of a lithium salt typically reducesthis elasticity down to a reversible tensile strain from approximately10% to 300%.

Several series of Li metal-sulfur and Li-ion sulfur cells were preparedusing the presently 5 prepared sulfonated elastomer composites as a2^(nd) anode-protecting layer and/or a cathode-protecting layer. Thefirst series was a Li metal cell containing an anode-protecting layerdisposed between a PE/PP-based porous separator and the Li metal foil(protected/covered by a 1^(st) anode-protecting layer) and the secondseries was also a Li metal cell having no anode-protecting layer. Thethird series was a Li-ion cell having a nanostructured anode ofconductive filaments (based on electrospun carbon fibers coated with athin layer of Si using CVD, protected by a graphene-based 1^(st)anode-protecting layer) plus an elastomer-based 2^(nd) anode-protectinglayer between this Si-based anode active material layer and a porousseparator. The fourth series was a Li-ion cell similar to the thirdseries, but without an anode-protecting layer. All these cells were madeinto two groups: one group containing a cathode-protecting layer and theother no cathode-protecting layer.

We have found that after large numbers of charge/discharge cycles, thecells containing anode-protecting layers were essentially dendrite-free.Such anode-protecting layers also appear to reduce the formation of deadlithium particles that are separated from the Li foil; this implies amore stable lithium-electrolyte interface. The presence of acathode-protecting layer has a dramatic impact on improving thecycle-life of an alkali metal-S cell by essentially eliminating theshuttle effect.

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof cathode active material, conductive additive or support, binder, andany optional additive combined). The specific charge capacity refers tothe amount of charges per unit mass of the composite cathode. Thespecific energy and specific power values presented in this section arebased on the total cell weight. The morphological or micro-structuralchanges of selected samples after a desired number of repeated chargingand recharging cycles were observed using both transmission electronmicroscopy (TEM) and scanning electron microscopy (SEM).

The cycling behaviors of 4 cells (each having a Li foil anode and acathode containing graphene-supported sulfur (S/graphene) particles asthe cathode active material) are shown in FIG. 3, which indicates that asulfonated PU/graphene composite-based cathode-protecting layer is veryeffective in extending the cycle-life of a Li—S cell (likely due to thereduction or elimination of the shuttle effect). The anode-protectinglayers make a more efficient use of lithium metal by reducing the amountof dead Li particles (these Li particles are physically separated fromthe Li metal foil and no longer active). Amon these 4 Li—S cells, acombination of two anode-protecting layers and a cathode-protectinglayer provides the most stable cycling response.

Example 17: Sulfonated SBS Elastomer Films as Electrode-ProtectingLayers and Graphene/Sulfur Particles as a Cathode Active Material

Tensile testing was also conducted on the sulfonated SBS elastomer films(without hybrid cathode particles). This series of elastomers can beelastically stretched up to approximately 230% (having some lithium saltdispersed therein).

Shown in FIG. 4 are the cycling behaviors of 3 Li—S cells; the firstcell has a cathode-protecting layer containing a sulfonated SBS (noanode-protecting layer), the second layer contains 2 anode-protectinglayers (no cathode-protecting layer), and the third cell has nocathode-protecting layer. The sulfonated elastomer-basedcathode-protecting layer has imparted cycle stability to the Li—S cellin a dramatic manner. However, the double anode-protecting layerapproach provides the most stable cycling behavior.

Example 18: Sulfonated PB Elastomer Composite-Based Protecting Layersand Sulfur-Impregnated Activated MCMB Particles as the Cathode ActiveMaterials

FIG. 5 shows the cycling behavior of three room-temperature Na—S celleach featuring a cathode active material layer containing sulfur-MCMB(activated) composite particles as the cathode active material: firstcell has a SIBS/RGO composite-based cathode-protecting protecting layer,second cell has 2 anode-protecting layers and 1 cathode protecting layer(SIBS/RGO), and the third cell has t no protecting layer. Again, theelastomer composite-based cathode-protecting layer has significantlyimproved the cycle stability to the Na—S cell. The shuttling effectcommonly associated with Li—S or Na—S cells has been significantlyreduced or essentially eliminated by the presently invented elastomercomposite-based cathode-protecting layer approach. The twoanode-protecting layers have significantly reduced the amount of dead Naparticles and eliminated the Na dendrite.

Example 19: Effect of Lithium Ion-Conducting Additive in a SulfonatedElastomer Composite

A wide variety of lithium ion-conducting additives were added to severaldifferent elastomers to prepare electrode-protecting layers. We havediscovered that these composite materials are suitable protecting layers(preventing dendrite formation in the anode side andreducing/eliminating the shuttle effect at the cathode side (notallowing S or metal polysulfide to diffuse out of the cathode), yetstill allowing reasonable alkali metal migration rates) provided thattheir lithium ion conductivity at room temperature is no less than 10⁻⁶S/cm. With these materials, lithium ions appear to be capable of readilydiffusing in and out of the protecting layer having a thickness nogreater than 1 μm. For thicker layers (e.g. 10 μm), a lithium ionconductivity at room temperature no less than 10⁻⁴ S/cm would berequired.

TABLE 1 Lithium ion conductivity of various sulfonated elastomercomposite compositions as a shell material for protecting anode activematerial particles. Graphene-sulfonated elastomer (1-2 μm thick); SampleLithium-conducting 5% graphene unless No. additive otherwise notedLi-ion conductivity (S/cm) E-1s Li₂CO₃ + (CH₂OCO₂Li)₂ 70-99%polyurethane, 4.8 × 10⁻⁶ to 4.9 × 10⁻³ S/cm 2% RGO E-2s Li₂CO₃ +(CH₂OCO₂Li)₂ 65-99% polyisoprene, 2.3 × 10⁻⁵ to 7.8 × 10⁻⁴ S/cm 8%pristine graphene E-3s Li₂CO₃ + (CH₂OCO₂Li)₂ 65-80% SBR, 15% RGO 8.8 ×10⁻⁶ to 8.9 × 10⁻⁴ S/cm D-4s Li₂CO₃ + (CH₂OCO₂Li)₂ 70-99% urethane-urea,2.4 × 10⁻⁶ to 7.3 × 10⁻⁴ S/cm 12% nitrogenated graphene D-5s Li₂CO₃ +(CH₂OCO₂Li)₂ 75-99% polybutadiene 2.4 × 10⁻⁵ to 7.9 × 10⁻³ S/cm B1sLiF + LiOH + Li₂C₂O₄ 80-99% chloroprene 2.2 × 10⁻⁶ to 6.4 × 10⁻⁴ S/cmrubber B2s LiF + HCOLi 80-99% EPDM 5.7 × 10⁻⁶ to 4.8 × 10⁻³ S/cm B3sLiOH 70-99% polyurethane 4.5 × 10⁻⁵ to 5.3 × 10⁻³ S/cm B4s Li₂CO₃ 70-99%polyurethane 6.3 × 10⁻⁵ to 5.8 × 10⁻³ S/cm B5s Li₂C₂O₄ 70-99%polyurethane 1.6 × 10⁻⁵ to 2.7 × 10⁻³ S/cm B6s Li₂CO₃ + LiOH 70-99%polyurethane 3.6 × 10⁻⁵ to 5.9 × 10⁻³ S/cm C1s LiClO₄ 70-99%urethane-urea 5.3 × 10⁻⁵ to 4.6 × 10⁻³ S/cm C2s LiPF₆ 70-99%urethane-urea 5.5 × 10⁻⁵ to 1.8 × 10⁻³ S/cm C3s LiBF₄ 70-99%urethane-urea 3.4 × 10⁻⁵ to 4.8 × 10⁻⁴ S/cm C4s LiBOB + LiNO₃ 70-99%urethane-urea 8.2 × 10⁻⁶ to 3.2 × 10⁻⁴ S/cm S1s Sulfonated polyaniline85-99% SBR 9.3 × 10⁻⁶ to 9.4 × 10⁻⁴ S/cm S2s Sulfonated SBR 85-99% SBR8.6 × 10⁻⁶ to 6.5 × 10⁻⁴ S/cm S3s Sulfonated PVDF 80-99%chlorosulfonated 5.4 × 10⁻⁶ to 5.8 × 10⁻⁴ S/cm polyethylene (CS-PE) S4sPolyethylene oxide 80-99% CS-PE 6.6 × 10⁻⁶ to 4.7 × 10⁻⁴ S/cm

Example 20: Cycle Stability of Various Rechargeable Lithium BatteryCells

FIG. 6 and FIG. 7 indicate that the presence of two anode-protectinglayers does not compromise the energy density of an alkali metal-sulfurcell even though the elastomer layer is normally less ion-conductingthan a liquid electrolyte. Quite unexpectedly, the energy density of thecell is actually improved, defying the expectations of materialsscientists.

The following observations can be made from the present study thatcovers a broad array of anode-protecting layers and cathode-protectinglayers:

-   -   1) The presently invented double protective layer approach for        the lithium anode enables the Li—S, Na—S, and K—S batteries to        deliver high cycling stability or long cycle life.    -   2) The invented approach also leads to alkali metal-sulfur        batteries having exceptional energy densities and power        densities. A cell-level energy density as high as 602 Wh/kg has        been achieved with Li—S cells.    -   3) Similar advantageous features are also observed with Na—S        cells and K—S cells. This is evidenced by FIG. 7, which shows        the Ragone plots (cell power density vs. cell energy density) of        4 alkali metal-sulfur cells.    -   4) The lithium dendrite or sodium dendrite issue is also        suppressed or eliminated.

In summary, the present invention provides an innovative, versatile, andsurprisingly effective platform materials technology that enables thedesign and manufacture of superior alkali metal-sulfur rechargeablebatteries. The alkali metal-sulfur cell featuring two protective layersat the anode and at least one protective layer at the cathode sideexhibits a high cathode active material utilization rate, high specificcapacity, high specific energy, high power density, little or noshuttling effect, and long cycle life.

1. A rechargeable alkali metal-sulfur cell selected from lithium-sulfurcell, sodium-sulfur cell, or potassium-sulfur cell, said alkalimetal-sulfur cell comprising: (a) an anode; (b) a cathode activematerial layer, comprising a sulfur-containing material selected from asulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid,conducting polymer-sulfur hybrid, metal sulfide, sulfur compound, or acombination thereof, and an optional cathode current collectorsupporting said cathode active material layer; and (c) an electrolyte oran electrolyte/separator layer; wherein said anode comprises: i) ananode active material layer containing a layer of lithium, sodium,potassium, a lithium alloy, a sodium alloy, a potassium alloy, alithium-absorbing compound, a sodium-absorbing compound, or apotassium-absorbing compound in a form of a foil, coating, or multipleparticles aggregated together, as an anode active material and anoptional anode current collector supporting said anode active materiallayer; ii) a first anode-protecting layer comprising a thin layer of anelectron-conducting material selected from graphene sheets, carbonnanotubes, carbon nanofibers, carbon or graphite fibers, expandedgraphite flakes, metal nanowires, conductive polymer fibers, or acombination thereof, wherein said first anode-protecting layer is inphysical contact with the anode active material layer and has athickness from 1 nm to 100 μm and a specific surface area greater than50 m²/g; and iii) a second anode-protecting layer in physical contactwith said first anode-protecting layer, having a thickness from 1 nm to100 μm and comprising an elastomer having a fully recoverable tensileelastic strain from 2% to 1,000% and a lithium ion conductivity from10⁻⁴ S/cm to 5×10⁻² S/cm when measure at room temperature.
 2. Therechargeable alkali metal-sulfur cell of claim 1, wherein saidsulfur-carbon hybrid, sulfur-graphite hybrid, sulfur-graphene hybrid, orconducting polymer-sulfur hybrid is a mixture, blend, composite,chemically or physically bonded entity of sulfur or sulfide with acarbon, graphite, graphene, or conducting polymer material.
 3. Therechargeable alkali metal-sulfur cell of claim 1, wherein said elastomercomprises a material selected from sulfonated versions or non-sulfonatedversions of the group consisting of natural polyisoprene, syntheticpolyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butylrubber, styrene-butadiene rubber, nitrile rubber, ethylene propylenerubber, ethylene propylene diene rubber, metallocene-basedpoly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer,styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber,polyacrylic rubber, silicone rubber, fluorosilicone rubber,perfluoroelastomers, polyether block amides, chlorosulfonatedpolyethylene, ethylene-vinyl acetate, thermoplastic elastomer, proteinresilin, protein elastin, ethylene oxide-epichlorohydrin copolymer,polyurethane, urethane-urea copolymer, and combinations thereof.
 4. Therechargeable alkali metal-sulfur cell of claim 1, wherein said graphenesheets are selected from the group consisting of pristine graphene,graphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, nitrogenated graphene, hydrogenated graphene, doped graphene,functionalized graphene, and combinations thereof.
 5. The rechargeablealkali metal-sulfur cell of claim 1, wherein said graphene sheetscomprise single-layer graphene or few-layer graphene, wherein saidfew-layer graphene is defined as a graphene platelet formed of less than10 graphene planes.
 6. The rechargeable alkali metal-sulfur cell ofclaim 1, wherein said graphene sheets have a length or width from 5 nmto 5 μm.
 7. The rechargeable alkali metal-sulfur cell of claim 1,wherein said metal sulfide contains M_(x)S_(y), wherein x is an integerfrom 1 to 3 and y is an integer from 1 to 10, and M is a metal elementselected from an alkali metal, an alkaline metal selected from Mg or Ca,a transition metal, a metal from groups 13 to 17 of the periodic table,or a combination thereof.
 8. The rechargeable alkali metal-sulfur cellof claim 7, wherein said metal element M is selected from the groupconsisting of Li, Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, and Al.
 9. Therechargeable alkali metal-sulfur cell of claim 1, wherein said metalsulfide comprises Li₂S₁, Li₂S₂, Li₂S₃, Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇,Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S₁, Na₂S₂, Na₂S₃, Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇,Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S₁, K₂S₂, K₂S₃, K₂S₄, K₂S₅, K₂S₆, K₂S₇, K₂S₈,K₂S₉, or K₂S₁₀.
 10. The rechargeable alkali metal-sulfur cell of claim1, wherein said carbon or graphite material in said cathode activematerial layer is selected from the group consisting of mesophase pitch,mesophase carbon, mesocarbon microbead (MCMB), coke particle, expandedgraphite flake, artificial graphite particle, natural graphite particle,highly oriented pyrolytic graphite, soft carbon particle, hard carbonparticle, carbon nanotube, carbon nanofiber, carbon fiber, graphitenanofiber, graphite fiber, carbonized polymer fiber, activated carbon,carbon black, and combinations thereof.
 11. The rechargeable alkalimetal-sulfur cell of claim 1, wherein said conducting polymer-sulfurhybrid contains an intrinsically conductive polymer selected frompolyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer,a sulfonated derivative thereof, or a combination thereof.
 12. Therechargeable alkali metal-sulfur cell of claim 1, wherein said elastomerfurther contains from 0.1% to 40% by weight of a lithium ion-conductingadditive or sodium ion-conducting additive dispersed therein.
 13. Therechargeable alkali metal-sulfur cell of claim 12, wherein said lithiumion-conducting additive is selected from the group consisting of Li₂CO₃,Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), Na₂CO₃, Na₂O, Na₂C₂O₄, NaOH, NaX,ROCO₂Na, HCONa, RONa, (ROCO₂Na)₂, (CH₂OCO₂Na)₂, Na₂S,Na_(x)SO_(y), andcombinations thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group,0≤x≤1 and 1≤y≤4.
 14. The rechargeable alkali metal-sulfur cell of claim12, wherein said lithium ion-conducting additive is selected fromlithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate(LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate(LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethylsulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.
 15. The rechargeable alkalimetal-sulfur cell of claim 12, wherein said lithium ion-conductingadditive comprises a lithium ion- or sodium ion-conducting polymerselected from the group consisting of poly(ethylene oxide) (PEO),polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methylmethacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, and combinations thereof.
 16. The rechargeablealkali metal-sulfur cell of claim 1, wherein said cell has a sulfurutilization efficiency from 80% to 99%.
 17. The rechargeable alkalimetal-sulfur cell of claim 1, wherein said electrolyte is selected frompolymer electrolyte, polymer gel electrolyte, composite electrolyte,ionic liquid electrolyte, organic liquid electrolyte, solid-stateelectrolyte, or a combination thereof.
 18. The rechargeable alkalimetal-sulfur cell of claim 1, wherein said electrolyte contains a saltselected from the group consisting of lithium perchlorate (LiClO₄),lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate(LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂,lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-methanesulfonate (NaCF₃SO₃),potassium trifluoro-methanesulfonate (KCF₃SO₃), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide(NaTFSI), bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), andcombinations thereof.
 19. The rechargeable alkali metal-sulfur cell ofclaim 1, wherein said electrolyte contains a solvent selected from thegroup consisting of ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma.-butyrolactone(γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF),methyl formate (MF), toluene, xylene or methyl acetate (MA),fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethylcarbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, room temperature ionicliquid, and combinations thereof.
 20. The rechargeable alkalimetal-sulfur cell of claim 1, wherein said anode active material layercomprises an anode active material selected from the group consisting oflithium metal, sodium metal, potassium metal, a lithium metal alloy,sodium metal alloy, potassium metal alloy, a lithium intercalationcompound, a sodium intercalation compound, a potassium intercalationcompound, a lithiated compound, a sodiated compound, a potassium-dopedcompound, lithiated titanium dioxide, lithium titanate, lithiummanganate, a lithium transition metal oxide, Li₄TisO₁₂, and combinationsthereof.
 21. The rechargeable alkali metal-sulfur cell of claim 1,further comprising one or two cathode-protecting layers disposed betweensaid cathode active material layer and the electrolyte orseparator/electrolyte layer wherein said cathode-protecting layers areselected from: A) a first cathode-protecting layer having a thicknessfrom 1 nm to 100 μm and comprising a thin layer of electron-conductingmaterial selected from graphene sheets, carbon nanotubes, carbonnanofibers, carbon or graphite fibers, expanded graphite flakes, metalnanowires, conductive polymer fibers, or a combination thereof, whereinsaid first anode-protecting layer has a specific surface area greaterthan 50 m²/g and is in physical contact with the cathode active materiallayer or the separator/electrolyte layer; and/or B) a secondcathode-protecting layer in physical contact with the firstcathode-protecting layer, having a thickness from 1 nm to 100 μm andcomprising an elastomer having a fully recoverable tensile elasticstrain from 2% to 1,000% and a lithium ion conductivity from 10⁻⁸ S/cmto 5×10⁻² S/cm when measure at room temperature.
 22. The rechargeablealkali metal-sulfur cell of claim 1, wherein said cell is a lithiumion-sulfur cell and said anode active material comprises alithium-absorbing compound selected from the group consisting of: (a)silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co),manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd), andlithiated versions thereof, (b) alloys or intermetallic compounds of Si,Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiatedversions thereof, wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (c) oxides, carbides, nitrides, sulfides,phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, andlithiated versions thereof, (d) salts and hydroxides of Sn and lithiatedversions thereof; (e) carbon or graphite materials and prelithiatedversions thereof, and combinations thereof.
 23. The rechargeable alkalimetal-sulfur cell of claim 1, wherein said cell is a sodium ion-sulfurcell or potassium ion-sulfur cell and said anode active materialcomprises a sodium-absorbing or potassium-absorbing compound selectedfrom the group consisting of: (a) sodium- or potassium-doped silicon(Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi),zinc (Zn), aluminum (Al), titanium (Ti), cobalt (Co), nickel (Ni),manganese (Mn), cadmium (Cd), and mixtures thereof, (b) sodium- orpotassium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c) sodium-or potassium-containing oxides, carbides, nitrides, sulfides,phosphides, selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof,(d) sodium or potassium salts; (e) particles of graphite, hard carbon,soft carbon or carbon particles and pre-sodiated versions thereof, and(f) combinations thereof.